What causes Gmail's 'sender's email address uses abnormal characters' error?

Key findings

• Character set issues: The primary cause is often the inclusion of characters that are not typically expected in an email address, such as those from non-Latin alphabets (like Cyrillic characters that resemble Latin ones) or other obscure Unicode characters.
• Homoglyph detection: Gmail's systems are sophisticated enough to detect homoglyphs, which are characters that look visually identical or very similar to common Latin characters but belong to different character sets. These can be exploited in phishing attempts to deceive recipients about the sender's true identity.
• Syntax validation: The error may also be triggered by subtle violations of RFC standards for email address syntax (RFC 5322), even if the characters themselves are standard. This could include issues with domain parts, local parts, or special characters used incorrectly.
• Security measure: This warning is a security mechanism by Gmail to protect users from deceptive practices like spoofing and phishing, where attackers try to impersonate legitimate senders.

Key considerations

• Inspect sender address: Thoroughly review the sender's email address for any unusual characters that might not be immediately obvious. This includes checking for Unicode variations or invisible characters. For more on character issues, read about invalid characters in email subject lines.
• Validate email address format: Ensure that the email address strictly adheres to RFC 5322 specifications for email message format, even for subtle deviations that might pass some validation checks but fail others.
• Avoid IDNs for primary sending: While Internationalized Domain Names (IDNs) are supported, their use in the sender's email address can sometimes trigger such warnings, especially if the recipient's email client or server has older character set handling.
• Review authentication: Although not directly related to the character error, strong email authentication (SPF, DKIM, DMARC) is crucial for overall deliverability and sender reputation. Check out a simple guide to DMARC, SPF, and DKIM to ensure your setup is robust. This helps improve sender trust and can indirectly mitigate warnings.

Key opinions

• Direct interpretation: Marketers tend to take Gmail's error message at face value, believing it literally means there are strange characters in their sender ID. They often confirm their visible sender ID appears normal, leading to puzzlement.
• SPF/DKIM confusion: There's a common initial thought that it might be related to SPF or DKIM, even though the error message doesn't explicitly mention authentication. This highlights the broad scope of deliverability issues marketers often consider.
• Impact on campaigns: Such warnings directly impact email campaign performance, as recipients are less likely to open emails flagged as suspicious, even if they're legitimate. For broader concerns, see what Gmail's 'dangerous' alert means for deliverability.

Key considerations

• Trust and reputation: Email marketers need to prioritize maintaining a strong sender reputation to avoid such warnings. These alerts directly undermine trust and can lead to emails landing in spam folders, as discussed in reasons emails go to spam.
• Address transparency: It's vital for marketing emails to use sender addresses that are clearly recognizable and free of any characters that could be misconstrued as attempts to spoof or deceive.
• Proactive monitoring: Regular monitoring of email deliverability and any warnings issued by major mailbox providers like Gmail is essential for marketers to quickly identify and address issues.

Key opinions

• Clear distinction from authentication: Experts clarify that this specific warning is generally unrelated to SPF, DKIM, or DMARC failures. It pertains directly to the characters within the sender's email address itself, indicating a character set or encoding issue.
• Homoglyph risk: A significant concern is the use of homoglyphs (e.g., Cyrillic 'а' vs. Latin 'a', or 'I' vs. 'l') that visually mimic standard characters but are different underneath. Gmail's filters are designed to detect such deceptive practices.
• Syntax and encoding: The error can stem from violations of email address syntax rules or improper character encoding, especially when dealing with internationalized email addresses (EMAIs) or obscure Unicode characters.

Key considerations

• Detailed address inspection: It's critical to meticulously analyze the sender's email address at a technical level, looking beyond visual appearance for hidden characters or encoding issues. This might involve using tools that reveal the precise Unicode values of characters.
• RFC compliance: While not every character issue is a strict RFC violation, adhering to established email address syntax (like RFC 5322) significantly reduces the risk of such warnings. This extends to domains and local parts.
• International domain name caution: If using Internationalized Domain Names (IDNs), ensure proper Punycode conversion and consider potential compatibility issues with older systems or strict filters. Such domains can be more prone to misinterpretation if not handled correctly. For broader deliverability insights, explore this expert guide to improving email deliverability.

Key findings

• RFC 5322 compliance: RFC 5322 defines the standard format for email messages, including the syntax for email addresses. Deviations, even subtle ones, from this core specification can lead to parsing errors or flagging by recipient servers.
• Internationalized Email Addresses (EMAIs): RFC 6531 introduced support for non-ASCII characters in email addresses. While valid, the implementation and acceptance by all mail transfer agents (MTAs) and user agents (UAs) can vary, sometimes leading to compatibility issues or warnings.
• Unicode character handling: Email systems must correctly handle various Unicode characters. Errors can arise if a sender uses a character that is visually ambiguous (a homoglyph) or if the encoding is not correctly interpreted by the recipient's system, leading to the 'abnormal characters' warning.
• Security best practices: Many email providers, including Gmail, implement additional security measures beyond RFC compliance to detect phishing and spoofing. These often involve heuristic analysis of sender names and addresses for suspicious character patterns or inconsistencies.

Key considerations

• Strict adherence to RFCs: Senders should ensure their email addresses strictly comply with RFC 5322 and other relevant RFCs concerning email syntax, such as RFC 2822. Even seemingly minor deviations can be flagged by modern email systems.
• Character encoding verification: Verify that all components of the email address (local part and domain) use widely accepted ASCII characters where possible to maximize compatibility and avoid triggering character-based alerts.
• Homoglyph awareness: Be aware of the potential for homoglyphs and intentionally avoid any characters that could be confused with others, particularly when constructing sender addresses for high-volume email. This proactive step helps prevent warnings and protect sender reputation. Further insights can be found in our guide to improving domain reputation.