Is it a good idea to put an email address in the subject line?
Michael Ko
Co-founder & CEO, Suped
Published 23 May 2025
Updated 15 Aug 2025
5 min read
Email subject lines are the first impression your message makes in a crowded inbox. They determine whether your email gets opened or ignored. As a result, crafting a compelling subject line is crucial for deliverability and engagement.
The question of whether to include an email address directly in the subject line often arises, particularly in scenarios like referral programs where identifying the sender might seem helpful. While the intent might be to enhance personalization or clarity, this practice carries significant risks.
From a deliverability perspective, putting an email address in the subject line can negatively impact how your email is perceived by inbox providers and ultimately, by recipients. It can trigger spam filters and erode trust, leading to lower open rates and potentially getting your sending domain added to a blocklist or blacklist.
The risk of including email addresses
One of the primary concerns with placing an email address in the subject line is its resemblance to spam. Spammers often employ deceptive tactics, including using random or personalized email addresses in subject lines, to trick recipients into opening their messages. Inbox providers like Gmail have sophisticated algorithms designed to detect these patterns.
When an email address appears in the subject, it can flag the email as suspicious, even if it comes from a legitimate sender. This increases the likelihood of the email landing in the spam folder rather than the inbox. This issue is particularly relevant given recent shifts in email sending requirements by major providers. For instance, emails with unusual subject line patterns may be seen as non-compliant with new authentication and spam prevention measures.
Beyond technical filtering, there's the human element. Many recipients are wary of emails that look like spam, and an email address in the subject line often raises a red flag. This can lead to lower engagement rates, as people might delete the email without opening it, or worse, mark it as spam.
Spam risk alert
Including an email address in your subject line can significantly increase the chances of your email being flagged as spam by various email providers, including Gmail and Outlook. This can lead to your legitimate messages being quarantined or rejected, severely impacting your email deliverability. Learn more about why Office 365 might block such emails.
The perceived benefit vs. the reality
Clients sometimes propose putting an email address in the subject line for programs where a recipient needs to know which friend sent them something. They might feel a simple name isn't enough to identify the specific sender, especially if multiple friends have similar names.
While this seems logical, it's a short-sighted approach. Most people recognize their friends by their full names rather than their email addresses. Including a full name, perhaps combined with a clear and concise message, is a far more effective and less risky personalization strategy.
Furthermore, if the sender's name is already displayed clearly in the From field, repeating the email address in the subject line becomes redundant and even counterproductive. It adds clutter and can dilute the message's impact. The key is to convey the message's purpose quickly and clearly without triggering spam filters.
Perceived benefit
Identification: Believed to help recipients identify which friend sent the message in a referral program, especially if names are common.
Clarity: Aims to provide immediate context about the sender to the recipient.
Actual impact
Spam triggering: High risk of emails being flagged by spam filters due to suspicious patterns.
Negative perception: Recipients often perceive it as lazy personalization or a spam attempt.
Reduced engagement: Leads to lower open rates and higher chances of being marked as spam or deleted without being read.
Email deliverability is a complex ecosystem, and every element of your email, including the subject line, contributes to your sender reputation. A poor subject line can lead to higher spam complaint rates and low engagement, which signals to ISPs that your emails are not valued by recipients. This can result in your emails being consistently directed to the spam folder, impacting future campaigns.
Maintaining a strong sender reputation is key to ensuring your emails reach the inbox. Putting an email address in the subject line undermines this goal. Instead of relying on such a risky tactic, focus on genuinely engaging content and proper email authentication protocols like SPF, DKIM, and DMARC.
For specific best practices on subject lines, research consistently shows that personalized, clear, and concise subject lines perform best. Mailchimp's guide on email subject line best practices emphasizes keeping them short and relevant to avoid spam filters and improve open rates. Similarly, Zeta Global highlights avoiding subject line mistakes that can lead to poor performance.
Good subject line practices
Bad subject line practices
Personalization: Use recipient's name (e.g., "John, your report is ready").
Email addresses: Avoid placing any email address (e.g., "friend@example.com sent you this").
Clarity & Conciseness: Clearly state the email's purpose. Keep it short (under 50 characters often works well).
Excessive punctuation/symbols: Avoid multiple exclamation marks, dollar signs, or unusual symbols, as these can trigger filters.
Value Proposition: Highlight a benefit or urgency (e.g., "Your order has shipped" or "Limited time offer").
Spammy keywords: Steer clear of words like "free" (unless used carefully), "win," or "guarantee" if they aren't directly relevant or appear suspicious. Learn about what words to avoid.
Testing: A/B test different subject lines to see what resonates with your audience.
Deceptive practices: Never use misleading information to boost open rates.
Prioritizing trust and deliverability
Ultimately, the best approach for subject lines is to focus on clarity, relevance, and personalization that feels authentic. While it might seem like a quick fix to identify a sender by their email address in the subject line, the long-term impact on your email deliverability and sender reputation far outweighs any perceived short-term gain.
Prioritize practices that build trust and ensure your emails consistently reach the inbox. If you're ever unsure about a subject line strategy, a small A/B test can provide valuable data, but for something as risky as including an email address, it's generally best to avoid it entirely.
Views from the trenches
Best practices
Use full names for personalization in subject lines, as this appears more legitimate.
Always disclose the actual sender in referral programs, adhering to regulatory requirements.
A/B test different subject line variations to understand what resonates best with your audience and deliverability.
Keep subject lines concise and clear, focusing on the email's core message.
Common pitfalls
Placing an email address in the subject line, which often triggers spam filters.
Assuming recipients recognize an email by its address in the subject line, rather than by name.
Ignoring the potential negative impact on domain and IP reputation from spam classifications.
Using deceptive or overly promotional language that can lead to low engagement or spam complaints.
Expert tips
Consider adding the full name of the referrer, e.g., 'John Smith sent you this link,' rather than an email address.
Spam filters are constantly evolving; what might have passed yesterday could be flagged today.
Focus on providing value and clear context in the subject line to encourage opens.
Monitor your deliverability metrics closely after any subject line changes.
Expert view
Expert from Email Geeks says that adding an email address in the subject line looks like lazy personalization or spam and suggests an A/B test for confirmation.
November 11, 2020 - Email Geeks
Marketer view
Marketer from Email Geeks believes that Gmail might interpret an email address in the subject line as spam because many spammers personalize by extracting the local part of the domain.
Gmail and 
Outlook. This can lead to your legitimate messages being quarantined or rejected, severely impacting your email deliverability. Learn more about why Office 365 might block such emails.
