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.

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.

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.

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.