What is a dotted hostname in the context of email and DNS records?
Michael Ko
Co-founder & CEO, Suped
Published 12 May 2025
Updated 19 Aug 2025
11 min read
When discussing email and DNS, terms can often be confusing. One phrase that sometimes comes up is "dotted hostname." It sounds technical and can lead to misunderstandings, especially when setting up critical email infrastructure. Many email deliverability professionals encounter this term when dealing with clients who might not be using standard domain configurations or who are referring to a very specific aspect of DNS.
The confusion often stems from the subtle but important distinctions between hostnames, domain names, and fully qualified domain names (FQDNs), and how the ubiquitous . (dot) character plays a role in each. While it might seem like a minor detail, accurately understanding and configuring these elements is crucial for ensuring your emails are delivered correctly and don't end up in spam folders or on a blacklist (or blocklist).
This guide will clarify what a "dotted hostname" typically refers to in the context of email and DNS records, why it matters for email deliverability, and how to ensure your domain configurations adhere to best practices to avoid common pitfalls.
Hostnames, domain names, and the dot
To understand a "dotted hostname," it's essential to first differentiate between a hostname and a domain name. A hostname is a human-readable label given to a specific device on a network, such as a server or computer. For instance, in mail.example.com, mail is the hostname. On the other hand, a domain name (like example.com) identifies a specific entity or organization on the internet.
The key to understanding "dotted hostnames" lies in the concept of a Fully Qualified Domain Name (FQDN). An FQDN is the complete and unambiguous address of a specific host on the internet. It includes both the hostname and the domain name, along with all subdomain labels, and traditionally ends with a dot (.) to signify the root of the DNS hierarchy. For example, mail.example.com. is an FQDN. The dot at the end is often omitted in everyday use, but it's technically implied, making the name absolute and fully qualified. You can learn more about FQDNs and their importance in this glossary entry.
The term "dotted hostname" isn't a widely recognized or official DNS term, which is why it can be confusing. Based on common discussions, it usually refers to a hostname that contains dots, effectively making it a subdomain or a multi-level subdomain. For instance, tracking.mail.example.com could be described as a "dotted hostname" if the user means tracking.mail is the specific server or service name within the example.com domain. This usage is less about the technical definition of a hostname and more about distinguishing it from a simple, single-label hostname (like server1). Every part of a domain name separated by dots is referred to as a label in DNS. Hostnames can contain multiple labels, but they are still part of a larger domain structure.
Dotted hostnames in email and DNS
In the context of email and DNS records, when someone refers to a "dotted hostname" not being a "real" domain (with SOA records), they are likely distinguishing between a subdomain that has its own DNS zone (delegated with NS records and having an SOA record) and a subdomain that is merely a hostname within a parent domain's DNS zone (having only A records, MX records, etc., but no delegated SOA). A hostname can indeed have various DNS records, such as A records mapping it to an IP address, or MX records for email routing. However, if it lacks its own NS or SOA records, it's essentially just a hostname (or subdomain) rather than a fully delegated domain.
The phrase "not according to RFC" (Request for Comments) refers to the official technical documents that define internet standards. For email, RFCs like RFC 5321 (SMTP) and RFC 5322 (Internet Message Format) dictate how mail servers and email addresses should behave. While a hostname with dots is perfectly valid for DNS resolution (e.g., server1.sub.example.com), using such a hostname directly as the 5322.From address in marketing emails is generally not recommended, even if technically permitted by some RFCs.
Understanding domain delegation
Domain delegation: When a subdomain (e.g., mail.example.com) has its own NS records pointing to separate nameservers, it means that subdomain's DNS records are managed independently. This creates a new DNS zone with its own SOA record.
Hostname within a domain: If a subdomain (or a multi-label name like news.mail.example.com) only has A records, MX records, etc., but no NS or SOA records, it's merely a host within the parent domain's DNS zone. The parent domain holds the authority for its records.
For email, the sending domain, whether it's a primary domain or a subdomain, needs proper DNS records including A records, PTR records, SPF, DKIM, and DMARC configured correctly. The underlying structure, whether a delegated subdomain or a simple hostname, needs to support these email authentication protocols for optimal deliverability.
Impact on email deliverability
The impact of a "dotted hostname" on email deliverability largely depends on how it's used and whether it's correctly configured within the DNS. While a hostname with multiple labels (dots) is not inherently problematic for DNS resolution, its use as a 5322.From address can raise deliverability concerns, especially for marketing emails.
Mailbox providers, like Gmail or Outlook, perform various checks to determine email legitimacy. These checks include verifying SPF, DKIM, and DMARC records, which rely on correctly configured DNS. If your "dotted hostname" isn't set up to align with these authentication protocols, it can significantly hinder your email deliverability. Improper DNS configurations can also lead to your domain (or IP) being added to an email blacklist (or blocklist), preventing your emails from reaching the inbox.
Typical hostname use
Server identification: Used for internal network identification or for specific services like mail.example.com for your mail server.
System messages: Occasionally used for system-generated emails or non-user facing communications.
Impact on email deliverability
Authentication issues: If not aligned with FQDNs for SPF, DKIM, and DMARC, emails may fail authentication checks. This can lead to emails landing in spam or being rejected. See our article about why your emails go to spam.
Sender reputation: Using non-standard or improperly configured sending domains can negatively impact sender reputation. This can result in emails being blocked or delayed.
Ultimately, for good email deliverability, it's about adhering to the established norms and technical requirements of DNS and email protocols. Using a full, properly set up domain or subdomain for your 5322.From address is almost always the best approach. If your client is referring to a domain that is not fully delegated (e.g., lacks SOA records), it's crucial to understand the implications for email and advocate for proper domain setup or subdomain delegation.
Best practices for email domains
To ensure robust email deliverability, especially when dealing with nuanced DNS configurations like those described as "dotted hostnames," focus on setting up your domains and hostnames according to established best practices.
Use FQDNs for email addresses: Always use a Fully Qualified Domain Name (FQDN) as the domain part of your 5322.From address. This means the domain used in your email address (e.g., @example.com) should be a properly registered domain or a delegated subdomain.
Configure essential DNS records: Ensure you have all necessary DNS records for email sending, including A, MX, SPF, DKIM, and DMARC. These records are vital for email authentication and signal to receiving mail servers that your emails are legitimate. You can learn more about DNS records for email.
Use subdomains for different streams: For better sender reputation management, consider using different subdomains for various email types (e.g., marketing.example.com, transactional.example.com). This isolates their reputations, so an issue with one doesn't affect the others. We have a guide on how to set up email subdomains.
When setting up DNS records, pay close attention to the syntax for various types. For example, a CNAME record often points to another FQDN, which might technically be seen as a "dotted hostname" if it's a multi-level subdomain. However, as long as the full path is specified and resolves correctly in DNS, it typically won't cause deliverability issues. The underlying problem highlighted by the client, about not using a "real" domain, suggests a fundamental misunderstanding or non-standard setup that needs to be addressed for reliable email sending.
Here is an example of common DNS records for an email sending subdomain:
The example above shows typical DNS records, where mail.example.com acts as a hostname for the mail server, but the primary email authentication records (MX, SPF, DKIM, DMARC) are typically set up at the root domain level or a delegated subdomain that functions as a full domain in itself. Note the trailing dots in the example, which signify Fully Qualified Domain Names (FQDNs). While many DNS providers automatically append the root, it's a good practice to understand their importance for unambiguous resolution, as explained on this DNS-SD page.
Navigating ambiguous terminology
It's worth noting that the term "dotted hostname" isn't standard across the industry, which can lead to confusion and miscommunication between parties. Clear communication regarding domain naming conventions and DNS setup is paramount to avoid deliverability issues. If a client insists on a non-standard configuration, it's vital to assess the risks and explain potential deliverability consequences.
While a hostname can have multiple dots (e.g., tracking.app.example.com), it generally only becomes a concern for email deliverability if it's not part of a larger, properly configured FQDN or if it lacks the necessary DNS records (like MX, SPF, DKIM) to support email sending and authentication. Ensuring your email sending domains, whether primary or subdomains, are fully compliant with DNS and email standards is the strongest defense against deliverability challenges.
Views from the trenches
Best practices
Always use a fully qualified domain name (FQDN) for your sending domain in email headers.
Ensure all necessary DNS records (A, MX, SPF, DKIM, DMARC) are properly configured and published.
Delegate subdomains (with NS and SOA records) if managing them as separate entities is necessary.
Common pitfalls
Using a hostname without corresponding MX, SPF, and DKIM records for email sending.
Misunderstanding the difference between a hostname, domain name, and a delegated subdomain.
Ignoring RFC compliance, which can lead to deliverability issues and being blocklisted.
Expert tips
Regularly monitor your domain's reputation and DNS record health to catch issues early.
Consult with DNS and email deliverability experts when encountering non-standard configurations.
Use dedicated subdomains for different email streams to protect your primary domain's reputation.
Expert view
Expert from Email Geeks says a hostname is valid for both the 5321.MailFrom and the 5322.From addresses, and can also have MX and SPF records, but using a hostname for the 5322.From is not recommended for anything other than system messages.
2020-01-17 - Email Geeks
Expert view
Expert from Email Geeks says if a domain only has an A record and no NS or SOA records, it is considered a hostname, regardless of the number of sub-domains in the FQDN.
2020-01-17 - Email Geeks
Conclusion
The term "dotted hostname" is somewhat ambiguous, but in the context of email and DNS, it often refers to a hostname that is effectively a subdomain with multiple labels (dots) within a larger domain structure, or potentially a non-delegated domain that lacks its own SOA records.
For optimal email deliverability, it's crucial to use fully qualified domain names (FQDNs) that are properly registered and configured with all necessary DNS records, including A, MX, SPF, DKIM, and DMARC. Adhering to these established standards ensures that your emails are authenticated and trusted by receiving mail servers, minimizing the risk of them landing in spam folders or being affected by blocklists. Clear communication and a solid understanding of DNS fundamentals are your best tools for navigating these complexities.
Gmail or 
Outlook, perform various checks to determine email legitimacy. These checks include verifying SPF, DKIM, and DMARC records, which rely on correctly configured DNS. If your "dotted hostname" isn't set up to align with these authentication protocols, it can significantly hinder your email deliverability. Improper DNS configurations can also lead to your domain (or IP) being added to an email blacklist (or blocklist), preventing your emails from reaching the inbox.
