Why do senders use single-character third-level domains in email?
Michael Ko
Co-founder & CEO, Suped
Published 15 Jun 2025
Updated 17 Aug 2025
6 min read
As email sending has evolved, so have the practices around domain naming. One intriguing pattern I've observed, particularly from larger senders and Email Service Providers (ESPs), is the consistent use of single-character third-level domains for sending email. You might have seen examples like e.example.com or m.brand.com. This raises a question: why opt for such a succinct structure, especially when longer, more descriptive subdomains are common?
Initially, it might seem counterintuitive, as clarity and explicit naming usually benefit domain reputation and human readability. However, there are several historical, technical, and strategic reasons that have led to the prevalence of these compact domain structures in the email ecosystem.
Historical roots of compact email domains
In the early days of the internet and email, bandwidth was a significant and costly resource. Every byte transmitted mattered, and this often translated into real money savings for large-volume senders and ESPs. Using shorter domain names, even by just a few characters, could accumulate into substantial cost reductions over millions or billions of emails.
Beyond cost, there was also a convention set by early Email Service Providers, such as Experian's CheetahMail, which started using e. as a prefix for email sending domains. This convention stuck and was adopted by others, establishing a familiar pattern in the industry. The single character e typically stood for 'email', providing a simple, quick identifier.
These historical practices explain why we continue to see these short domains today, even if the initial cost-saving impetus is less critical in an era of abundant bandwidth. Habits die hard in technology, especially when they've been ingrained for decades.
Technical drivers and display considerations
Beyond historical reasons, there were also technical considerations that made shorter domains attractive. The Domain Name System (DNS) primarily uses User Datagram Protocol (UDP) for queries, which has a 512-byte limit for responses. While DNS can fall back to Transmission Control Protocol (TCP) for larger responses, this introduces additional latency, as it requires establishing a new connection.
For systems handling millions of DNS queries for email sending and tracking, even a slight increase in latency per query can compound into significant delays. Keeping hostnames short helps ensure DNS responses fit within the UDP limit, optimizing query speed. While modern DNS implementations and network speeds have mitigated this to some extent, the practice of using short names persists.
Another factor is the desire for brevity and a cleaner appearance. A shorter domain can sometimes look visually cleaner in email clients or when displayed in tracking links. This can reduce the chance of truncation in some interfaces and potentially make the email address appear less cluttered to the recipient, although this is more subjective.
Historical drivers
Bandwidth costs: In the past, every character saved meant reduced data transfer costs, which was a significant operational expense for high-volume senders.
Early ESP conventions: Major Email Service Providers established e. as a standard for email subdomains, which was then widely adopted across the industry.
DNS efficiency: Shorter names helped DNS queries fit within UDP packet limits, avoiding slower TCP fallbacks for improved lookup speed.
Modern relevance
Legacy continuation: Many organizations continue using these conventions out of habit, or because their existing infrastructure is already configured this way.
Stream segmentation: Single-character subdomains like m. (marketing) or t. (transactional) still serve to separate email streams, which is crucial for reputation management.
Perceived aesthetic: A shorter, simpler domain can appear less cumbersome in an email address or link, potentially improving perceived trustworthiness.
Example DNS record for a single-character third-level domainDNS
e.example.com. 3600 IN A 192.0.2.1
Sender reputation and brand perception
When it comes to email deliverability, the impact of a single-character third-level domain is nuanced. While recipients might not consciously register the exact length or structure, mail servers (receiving mail transfer agents) primarily care about proper authentication and sender reputation. A short subdomain, by itself, is neither inherently good nor bad for deliverability.
What truly matters is how that subdomain is managed. If it's correctly configured with SPF, DKIM, and DMARC records, and if the sending practices associated with it are legitimate, its length is largely irrelevant. Mailbox providers like Google and Yahoo focus on these technical signals and the sending behavior tied to the domain and its subdomains.
From a branding perspective, a single-character subdomain like e. might not offer much descriptive value to a human recipient. However, its primary purpose is not human readability but rather to serve as a technical identifier for specific email streams. Businesses often use subdomains to segment different types of email (e.g., transactional, marketing, alerts) to protect their main domain's reputation in case one stream encounters deliverability issues.
Ensuring optimal deliverability with subdomains
While single-character subdomains have their historical and technical basis, their effectiveness today hinges on proper configuration and management. Here are key considerations:
DNS records: Always ensure that your DNS entries are correctly published and accessible, especially for authentication protocols like SPF, DKIM, and DMARC. Missing or misconfigured records can lead to deliverability problems.
Authentication standards: Implement SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance) for all sending domains and subdomains. These are critical for proving your emails are legitimate and protecting against spoofing.
Reputation management: Monitor the reputation of each subdomain separately. Segmenting email types to different subdomains helps isolate any negative impact from a problematic sending stream, preventing it from affecting your primary domain.
Modern perspectives on subdomain use
In today's email landscape, while the historical reasons for single-character third-level domains are less pressing, their strategic value for email segmentation and reputation management remains relevant. The concept of using subdomains to categorize and manage different email types (e.g., marketing, transactional, dedicated IP pools) is a widely accepted deliverability best practice.
Whether you choose a single-character subdomain or a more descriptive one like news.yourbrand.com depends on your specific needs and priorities. For some, the historical convention and perceived technical efficiency of a short subdomain may outweigh the benefits of a more descriptive name. For others, clarity and branding might lead them to choose longer, more explicit subdomains.
The key takeaway for senders is that consistent, legitimate sending practices and robust email authentication (SPF, DKIM, DMARC) are far more critical than the length of your third-level domain. Regular monitoring of DMARC reports and managing your blocklist status will always be paramount to deliverability success.
Automated system emails, password resets, order confirmations.
Support subdomain
s.yourdomain.com
Customer support communications, ticketing systems.
Tracking domain
click.yourdomain.com
Hosts redirects for tracking link clicks.
Views from the trenches
Best practices
Always properly authenticate your sending domains with SPF, DKIM, and DMARC to build sender trust, regardless of domain length.
Segment your email traffic by using different subdomains for different types of mail, such as marketing and transactional.
Regularly monitor your domain's reputation and DMARC reports to quickly identify and address any deliverability issues.
Common pitfalls
Relying solely on short domain names for perceived technical advantages that are now less significant in modern networks.
Failing to set up proper DNS records, including authentication, for all subdomains used for email sending.
Not understanding that while short domains might save minimal bytes, they don't inherently improve deliverability or spam filtering.
Expert tips
DNS queries for large DNS responses can fall back to TCP, which adds latency. Shorter hostnames can help keep queries within UDP limits for faster resolution.
Early ESPs established the 'e.' convention for email, which became a widely adopted standard in the industry.
The practice of using single-character third-level domains is largely a legacy of historical internet constraints, such as bandwidth costs and DNS packet sizes.
Marketer view
Marketer from Email Geeks says that senders try to be as short as possible, hoping the From address fits and links are less likely to truncate when displayed.
2024-07-01 - Email Geeks
Expert view
Expert from Email Geeks says the 'e.' convention was introduced by an early ESP, possibly Cheetah (Experian), around 2005, primarily for email.
2024-07-01 - Email Geeks
Key takeaways for email senders
The choice to use single-character third-level domains in email, such as e.yourdomain.com, is a practice rooted in the early days of email and internet infrastructure. Factors like bandwidth limitations and DNS efficiency played a role in their initial adoption, as did the conventions established by pioneering ESPs. While some of these technical constraints have lessened over time, the convention persists due to legacy systems and established practices.
Ultimately, the effectiveness of any sending domain, regardless of its length, comes down to proper configuration and responsible sending. Focusing on robust email authentication, segmenting email streams, and maintaining a healthy sender reputation will always yield the best deliverability outcomes.
Google and 
Yahoo focus on these technical signals and the sending behavior tied to the domain and its subdomains.
