What causes Gmail's 'sender's email address uses abnormal characters' error?
Matthew Whittaker
Co-founder & CTO, Suped
Published 12 May 2025
Updated 19 Aug 2025
7 min read
The message from Gmail, "sender's email address uses abnormal characters," can be quite puzzling for email senders. When you see this warning, it suggests that something is off with the email address formatting, and Gmail's systems have flagged it as potentially suspicious or even dangerous. This isn't just about a typo, it often points to deeper issues that could impact your email deliverability and sender reputation.
It's a common misconception that this error solely refers to literal, non-standard characters in the From address. While that's often the primary cause, especially with international characters or certain symbols, the warning can also be a blanket alert for other irregularities that make an email address seem unusual or designed to deceive. It's how Gmail attempts to protect users from potential phishing or spoofing attempts. Sometimes it's a false positive, but other times it's a clear indicator of a compliance problem. You can read more about how this general warning appears in the Gmail Help Community.
Identifying and resolving this issue is crucial for maintaining a healthy sender reputation and ensuring your messages reach the inbox reliably. Ignoring this warning can lead to emails being blocked, marked as spam, or even resulting in your domain being added to an email blacklist (or blocklist).
Anatomy of abnormal characters
The most straightforward reason for this error is the presence of characters in the sender's email address that do not conform to standard email address specifications, particularly RFC 5322. While modern email systems are becoming more flexible with international characters (such as Internationalized Domain Names, or IDNs), Gmail remains cautious due to the potential for abuse.
Certain characters, even if technically allowed by some relaxed interpretations of RFCs, can trigger Gmail's filters because they are frequently used in phishing or spam campaigns to obscure the true sender or mimic legitimate addresses. This includes non-ASCII characters, excessive special characters, or characters that look similar to standard English letters (homoglyphs).
For example, using Unicode characters or emojis in the From address, while visually distinct, can lead to this warning. Gmail is known to block emails for this specific reason because these characters can be abused to create deceptive sender identities.
Beyond the characters: underlying issues
While character sets are a direct cause, the Gmailsender's email address uses abnormal characters warning often points to deeper underlying issues. Gmail's filters are sophisticated, analyzing various signals beyond just the literal characters. These signals include sender reputation, email authentication, and content analysis.
A low sender reputation can cause Gmail to scrutinize your emails more heavily. If your domain has a history of sending spam or generating complaints, even a perfectly formatted email address might trigger a warning. Inadequate email authentication, such as missing or misconfigured SPF, DKIM, or DMARC records, can also contribute. Gmail uses these protocols to verify sender legitimacy, and their absence or incorrect setup can make your emails appear suspicious or like potential phishing attempts.
The warning might also be a symptom of email spoofing, where malicious actors try to send emails that appear to come from your domain. If Gmail detects such activity, it will display warnings to protect its users, even if your legitimate emails are properly formatted. This is especially true if emails fail DMARC authentication checks. In these cases, the abnormal characters message serves as a broader security alert. This is similar to why Gmail marks emails as phishing.
Literal character issues
Non-ASCII characters: Includes Unicode symbols, emojis, or characters from non-Latin alphabets in the local part or domain.
Invalid symbols: Using special characters that are explicitly disallowed by RFCs, beyond the standard dot, hyphen, or underscore.
Homoglyphs: Characters that visually resemble ASCII characters but are from a different character set, often used in spoofing.
Broader deliverability and security concerns
Poor sender reputation: If your domain has a history of high spam complaints or bounces, Gmail might flag even compliant addresses.
Authentication failures: Lacking or incorrectly configured SPF, DKIM, or DMARC records can make emails appear untrustworthy.
Spoofing attempts: If malicious actors are spoofing your domain, Gmail may issue warnings across the board to protect recipients.
Diagnosing and resolving the issue
Resolving this error requires a systematic approach. First, you need to verify the sender's email address itself. Ensure it strictly adheres to ASCII characters and avoids any unusual symbols, emojis, or Unicode characters. Even if your email service provider allows them, Gmail may still flag them as abnormal. Consider using a straightforward email address format.
Next, examine your email authentication records. Ensure your SPF, DKIM, and DMARC records are correctly set up and aligned. You can use a tool to check your DMARC record to ensure it is valid and properly configured. These measures help Gmail (and other providers like Yahoo!) verify your legitimacy and reduce the likelihood of your emails being flagged. In some cases, Gmail might even apply warnings if it perceives your domain is being incorrectly marking messages as spam.
It's also important to monitor your sender reputation using tools like Google Postmaster Tools. A declining reputation can trigger various warnings, including the abnormal characters message, as Gmail becomes warier of your sending practices. Addressing these foundational elements will significantly improve your email deliverability and reduce the likelihood of this particular error.
Best practices for email addresses
Stick to ASCII: Always use standard ASCII characters (a-z, A-Z, 0-9, and allowed symbols like '.', '-', '_') in both the local part and the domain of your email address.
Validate format: Regularly check that your sender email addresses conform to RFC 5322 standards. Automated validation can help prevent errors.
Maintain strong authentication: Implement and monitor SPF, DKIM, and DMARC to prove sender legitimacy and prevent spoofing.
Proactive measures and ongoing vigilance
If you're facing this Gmail warning, it's essential to investigate thoroughly. Start by meticulously checking the sender address for any non-standard characters, even subtle ones that might be easily overlooked. Remember that Gmail prioritizes user safety, and its algorithms are designed to err on the side of caution when an email address appears anything less than perfectly legitimate.
Beyond character checks, ensure your domain has strong email authentication in place. This includes correctly configured SPF, DKIM, and DMARC records. These are critical for building and maintaining a positive sender reputation with Gmail and preventing your legitimate emails from being flagged. Regularly checking blocklists and monitoring for any unusual activity related to your sending domain is also a good practice.
Views from the trenches
Best practices
Always use standard ASCII characters in your email addresses, including the local part and domain.
Regularly validate your email addresses against RFC standards to catch any non-compliant characters early.
Implement robust email authentication (SPF, DKIM, DMARC) to build trust with mailbox providers like Gmail.
Monitor your sender reputation and DMARC reports for any signs of spoofing or delivery issues.
Common pitfalls
Using Unicode characters, emojis, or non-standard symbols in email addresses, even if your platform allows it.
Neglecting email authentication, which makes your emails appear less trustworthy to strict filters.
Ignoring early warnings from Gmail, leading to a degraded sender reputation and stricter flagging.
Not checking for homoglyph attacks or lookalike domains that could be spoofing your legitimate address.
Expert tips
Examine your raw email headers for the exact sender address being transmitted, as sometimes platforms can alter it.
If your domain is relatively new or has low sending volume, Gmail might be more cautious about unusual formatting.
Be aware of how internationalized domain names (IDNs) are handled, as punycode conversion can sometimes introduce complexities.
Review your DMARC reports for forensic data that might indicate specific malformed addresses or spoofing attempts.
Marketer view
Marketer from Email Geeks says they often see the 'sender’s email address uses abnormal characters' error from Gmail.
2023-06-27 - Email Geeks
Marketer view
Marketer from Email Geeks says they suspected SPF was the issue, but the error message did not seem to match that reasoning.
2023-06-27 - Email Geeks
Summary: keys to avoiding this Gmail warning
The Gmailsender's email address uses abnormal characters error is more than just a minor formatting glitch, it's a critical signal from Gmail that your email sending practices might be perceived as suspicious. While directly related to email address format, it often hints at broader issues like poor sender reputation or authentication failures.
To effectively address this warning, ensure your sender email addresses strictly adhere to ASCII standards, strengthen your email authentication with robust SPF, DKIM, and DMARC policies, and proactively monitor your sender reputation. By taking these steps, you can improve your email deliverability and avoid falling victim to Gmail's protective filters.
attempts to protect users from potential phishing or spoofing attempts. Sometimes it's a false positive, but other times it's a clear indicator of a compliance problem. You can read more about how this general warning appears in the Gmail Help Community.
remains cautious due to the potential for abuse.
Yahoo!) verify your legitimacy and reduce the likelihood of your emails being flagged. In some cases, Gmail might even apply warnings if it perceives your domain is being incorrectly marking messages as spam.
