Why do senders use single-character third-level domains in email?

9 Marketer opinions 10 Expert opinions 4 Technical articles 3 Resources

Summary

Senders adopted single-character third-level domains primarily for historical reasons linked to technical and economic constraints. Shorter domain names, such as 'e.domain.com,' helped reduce character counts in email addresses and URLs, preventing truncation and fitting within display limits. This also lowered bandwidth costs significantly when T1 lines were expensive. ESPs like Experian (CheetahMail) and Epsilon popularized the convention, using 'e' for email, 't' for transactional, and 's' for support. Shorter hostnames ensured DNS responses fit within 512 bytes, preventing slower TCP fallback. While the relevance of single-character subdomains has diminished with improved infrastructure and reduced bandwidth costs, the broader practice of using subdomains for segmenting email traffic, protecting sender reputation, and enhancing deliverability remains valuable. These strategies help in separating email streams and maintaining organizational clarity.

Key findings

Historical Context: Single-character subdomains emerged due to historical technical and economic constraints.
Bandwidth Costs: Shorter domain names significantly reduced bandwidth costs when bandwidth was expensive (T1 lines).
Character Limits: Shorter addresses ensured they fit in display locations and reduced truncation risk.
DNS Optimization: Smaller hostnames ensured DNS responses fit within 512 bytes, avoiding TCP fallback.
ESP Conventions: Early ESPs like Experian (CheetahMail) and Epsilon established naming conventions such as 'e' for email.
Reputation Management: Subdomains help protect sender reputation and improve email deliverability by segmenting email traffic.
FQDN Locations: FQDNs (Fully Qualified Domain Names) display in email addresses, "show me the link", and landing page URLs.
Modern Relevance: Though less crucial now, subdomains remain important for organization and deliverability.

Key considerations

Legacy Considerations: The historical reasons for single-character subdomains are less critical with modern infrastructure.
Subdomain Strategy: Develop a subdomain strategy that aligns with current best practices for email deliverability and sender reputation.
Alternative Naming: Consider descriptive subdomains (e.g., 'email.domain.com', 'updates.domain.com') while maintaining brevity.
Latency: UDP requests are faster and will have lower latency than TCP requests.
Domain Aliases: Domain aliases can provide similar functionality in terms of separation for specific email streams or purposes.
Email Authenticity: Ensure correct DNS configurations, including SPF records, for email authenticity and deliverability.

What email marketers say
9Marketer opinions

Senders use single-character third-level domains in email for a variety of reasons, primarily rooted in historical practices. Originally, 'e' often signified 'email,' and shorter domains helped minimize character counts when limits were more restrictive and bandwidth was expensive. This brevity also assisted in click and open rate tracking via shorter URLs. While not as critical today, the use of subdomains in general remains a beneficial strategy for segmenting email traffic, protecting sender reputation, and improving deliverability by separating different types of email streams (e.g., transactional vs. marketing). Shorter domain names can also avoid DNS truncation issues, ensuring faster lookups. Though legacy in some aspects, the concept of subdomains for organizational and deliverability purposes remains highly relevant.

Key opinions

Historical Origins: Single-character domains began as a way to shorten URLs for tracking and to save bandwidth when it was a significant cost factor.
Bandwidth Savings: Shorter domains potentially saved 2-3% on bandwidth costs in the past.
DNS Considerations: Short domain and subdomain names can avoid DNS truncation issues and forced TCP fallback.
Reputation Management: While single-character domains may be less common, using subdomains generally helps protect sender reputation and improve email deliverability by separating email streams.
Character Limits: Single-character domains were useful when character limits were more stringent.
Email Indication: The letter 'e' has long been associated with indicating email services.

Key considerations

Relevance Today: Single-character domains are less critical today with increased bandwidth and fewer character restrictions, but the broader concept of subdomains remains important.
Subdomain Strategy: Develop a subdomain strategy that aligns with current best practices for email deliverability and sender reputation, rather than solely relying on single-character domains.
Alternative Naming: Consider using more descriptive subdomains (e.g., 'email.domain.com', 'updates.domain.com') while still maintaining brevity where possible for DNS efficiency.
Long Term Cost: With bandwidth no longer a large cost consider if you want to use longer subdomain names to make them more identifiable for users or anti-spam purposes.

Marketer view

Email marketer from Reddit user u/EmailPro shares that single-character subdomains were a legacy practice, used when character limits were more restrictive. They suggest it is less relevant today.

August 2024 - Reddit

Marketer view

Email marketer from NeilPatel.com shares that using subdomains (like 'e.domain.com') helps segment email traffic, improving sender reputation. While not exclusively single-character, this highlights the broader strategy of subdomain usage for deliverability.

May 2022 - NeilPatel.com

Marketer view

Email marketer from Mailjet shares that subdomains (like 'email.yourdomain.com') help protect your main domain's reputation. Although not always single character, this practice extends the best practice and provides some of the same benefits.

February 2024 - Mailjet

Marketer view

Marketer from Email Geeks explains that 'e' usually stands for email.

November 2021 - Email Geeks

Marketer view

Email marketer from StackExchange user TechGuru mentions subdomain use for organizational purposes and improved deliverability. Although not specifically one character, it follows the correct methodology.

June 2023 - StackExchange

Marketer view

Email marketer from Sendgrid shares that dedicated IPs and subdomains are used to protect a companies main sending reputation and therefore deliverability. While it doesn't recommend single characters, it shows the need for separation of concerns.

February 2024 - Sendgrid

Marketer view

Email marketer from AnEmailForum called OldTimer says that single letter subdomains use to be for bandwidth reasons. He states that it could have saved as much as 2-3% on bandwidth costs.

January 2025 - AnEmailForum

Marketer view

Email marketer from Quora user Jane Doe shares that single-letter subdomains began with early ESPs wanting short URLs to track clicks and open rates. It became a historical practice.

September 2022 - Quora

Marketer view

Email marketer from DNSDiscussions called DNSexpert mentioned that having short domain and subdomain names can help avoid problems with DNS truncation where TCP becomes enabled.

February 2023 - DNSDiscussions

What the experts say
10Expert opinions

Senders historically employed single-character third-level domains to minimize length for various reasons. Shorter addresses ensured they fit in display locations, reduced the likelihood of truncation, and, most importantly, decreased data transmission costs when bandwidth was expensive (T1 lines costing significantly more). ESPs, such as Experian (CheetahMail) and Epsilon, popularized conventions like 'e' for email, 't' for transactional, and 's' for support around 2005. Smaller hostnames also helped ensure DNS responses fit within 512 bytes, preventing fallback to slower TCP connections. These practices were common two decades ago when infrastructure and network conditions differed vastly from today.

Key opinions

Length Minimization: Addresses were shortened to fit display locations and reduce truncation risk.
Cost Savings: Shorter hostnames significantly reduced bandwidth costs when bandwidth was expensive.
DNS Optimization: Smaller hostnames ensured DNS responses fit within 512 bytes, avoiding TCP fallback.
ESP Conventions: Early ESPs like Experian (CheetahMail) and Epsilon established naming conventions such as 'e' for email.
Historical Context: These practices were most relevant in the early 2000s when network conditions were different.
FQDN Location: FQDNs (Fully Qualified Domain Names) display in email addresses, "show me the link", and landing page URLs.

Key considerations

Legacy Practice: While these practices were once critical, modern infrastructure and bandwidth costs make them less essential.
Infrastructure: The practice was useful with the infrastructure constraints 20 years ago but may not be helpful or needed now.
Latency: UDP requests are faster and will have lower latency than TCP requests.

Expert view

Expert from Word to the Wise explains that shorter hostnames for some infrastructure can help ensure DNS responses fit in 512 bytes. That used to be a good practice.

March 2023 - Word to the Wise

Expert view

Expert from Word to the Wise shares a historical perspective that ESPs did the math on sending costs, and the extra bytes saved by smaller hostnames everywhere were shaving some noticeable fraction (2-3%?) off their bandwidth costs.

July 2023 - Word to the Wise

Expert view

Expert from Email Geeks shares that the 'e' as a convention was done by an early ESP, possibly Experian. Shorter was better as it saved actual money back in the day due to fewer characters to send.

August 2021 - Email Geeks

Expert view

Expert from Email Geeks shares they used it at Epsilon. 'e' for email, 't' for transactional, 's' for support. It's just a short unique subdomain, circa 2005.

January 2022 - Email Geeks

Expert view

Expert from Email Geeks shares that spinning up the TCP connection is noticeably more latency than flinging out a UDP request and waiting for a response.

June 2021 - Email Geeks

Expert view

Expert from Email Geeks explains that there were real money savings by dropping 3 or 4 letters out of hostnames. T1 lines were expensive back then.

November 2024 - Email Geeks

Expert view

Expert from Email Geeks explains that the FQDN would display anywhere including the email address, in a "show me the link", and the landing page URL.

November 2021 - Email Geeks

Expert view

Expert from Email Geeks shares that they still use short hostnames for some infrastructure to ensure DNS responses fit in 512 bytes. Back in the day, ESPs had done the math about sending costs, and the extra bytes saved by smaller hostnames everywhere were shaving some noticeable fraction (2-3%?) off their bandwidth costs.

October 2024 - Email Geeks

Expert view

Expert from Email Geeks explains that these conventions were established 20 years ago when the world was different.

August 2021 - Email Geeks

Expert view

Expert from Email Geeks explains the use of single-character third-level domains is to be as short as possible, hoping the from address fits, and hoping links are less likely to truncate when displayed.

December 2022 - Email Geeks

What the documentation says
4Technical articles

The documentation highlights the importance of domain and email configuration, and hints at the historical reasons for brevity. RFC 2142 provides a precedent for short mailbox names, potentially influencing domain naming. Google Workspace Admin Help mentions domain aliases, which can act as a separation, but doesn't talk about single character subdomains. Microsoft Learn describes SPF records that are important for email authenticity. Lastly RFC 1035 sets the standards for domain names, and their limits which gives context to why shorted names were important.

Key findings

RFC 2142 - Local Part Naming: RFC 2142 illustrates historical precedent for short designations in mailbox names.
RFC 1035 - Domain Length: RFC 1035 specifies the standards for domain names, including length restrictions.
SPF Records: SPF records are essential for defining authorized email sending sources.
Domain Aliases: Domain aliases allow sending and receiving email from other domains.

Key considerations

Context: While single-character subdomains aren't explicitly addressed, the documents provide context for related email and domain configurations and historic decisions.
Naming Conventions: Domain naming conventions and limits have changed over time which mean domain names should be re-evaluated.
Email Authenticity: Email authenticity and correct DNS configurations are important.

Technical article

Documentation from Google Workspace Admin Help explains that domain aliases allow you to send and receive email from another domain. Single-character subdomains are not mentioned but they may be used to achieve a similar function of separation of function.

December 2024 - Google Workspace Admin Help

Technical article

Documentation from Microsoft Learn shows how to configure Sender Policy Framework (SPF) records. SPF records include authorized sending sources. Although it is not about single characters, it shows the need for correct domain configuration.

January 2022 - Microsoft Learn

Technical article

Documentation from IETF (RFC 1035) specifies the standards for domain names, including length restrictions. While not directly about single-character subdomains, it provides context for why brevity was historically important in DNS.

June 2022 - IETF

Technical article

Documentation from IETF (RFC 2142) outlines common mailbox local-part names. While not directly related to third-level domains, it illustrates a historical precedent for short, single-character designations for mailboxes, potentially influencing domain naming conventions for email infrastructure.

October 2024 - IETF

third-level domain used for spoofing - Networking - Spiceworks ...

I just got off a call with Microsoft regarding an email with this From address: no-reply@accountreset.microsoft.com The guy I spoke to told me it’s possible to obtain a third-level domain without needing to own the underlying second-level domain. This doesn’t sound right to me. Like it sounds really wrong to me. Can someone comment on this?

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