Suped

Summary

Encountering 550 Subject contains invalid characters errors when sending emails can be a frustrating experience for any sender. Often, the subject line might appear perfectly normal in your email client, yet bounce back as invalid from recipient mail servers. This issue typically stems from subtle encoding problems, such as a hidden trailing null byte in the Base64 representation of the subject, which some mail transfer agents (MTAs) interpret as an invalid character.

What email marketers say

Email marketers often weigh the benefits of engaging subject lines, sometimes involving special characters or emojis, against potential deliverability risks. While some special characters can increase open rates, others, particularly those causing encoding issues, can lead to messages being blocked or flagged as spam. The consensus is to be cautious and test thoroughly.

Marketer view

Email marketer from Email Geeks notes that even carefully crafted subject lines can be altered at the recipient level, causing them to appear invalid. This suggests that the issue might not be with the initial creation but with how MTAs process or re-encode the header. A lack of pattern in the spam appliances involved makes diagnosis more challenging, implying the problem could be deeply rooted in encoding.

23 Sep 2020 - Email Geeks

Marketer view

A marketer from Practical Ecommerce suggests that using special characters in subject lines can be a double-edged sword. While they might increase open rates by standing out, they also carry the risk of deliverability issues if not handled correctly. It emphasizes the need for cautious implementation and testing.

10 Oct 2022 - Practical Ecommerce

What the experts say

Experts in email deliverability and systems administration often delve into the technical nuances of subject line encoding when diagnosing bounce issues. Their insights typically center around character sets, MIME encoding standards, and the precise bytes that constitute an email header, identifying subtle errors that can lead to rejections from even a few, but critical, mail servers.

Expert view

Expert U69M02SDU from Email Geeks explains that UTF-8 encoded subject lines are indeed valid if they stick to original (non-extended) ASCII characters. They suggest the true source of the issue might be the sender's MTA or ESP altering the encoding, rather than the recipient's system.

25 Sep 2020 - Email Geeks

Expert view

Expert UDEFP1J3B from Email Geeks identifies that the Base64 encoded subject line, when decoded, contains the original subject plus a trailing zero byte. This is a critical insight, pointing to a potential off-by-one error in the encoding software that might go unnoticed by most mail servers but not all.

25 Sep 2020 - Email Geeks

What the documentation says

Official documentation and technical specifications, particularly RFCs related to email standards, define how characters in subject lines should be handled. When errors like invalid characters occur, it often indicates a deviation from these prescribed methods, such as incorrect encoding or the inclusion of control characters where only displayable ones are expected.

Technical article

Documentation on the Spiceworks Community forum indicates that a 550 Subject contains invalid characters response directly points to an issue with the subject line's formatting or content as perceived by the receiving mail server. This often occurs even if the sender perceives the subject as valid, suggesting a mismatch in character interpretation or encoding standards between systems.

20 Feb 2020 - Spiceworks Community

Technical article

Aspose.com forum discussions highlight that when email files (EML, MSG) have subject lines containing special characters like <> or "", attachments might not be extracted. This suggests that invalid characters in the subject line can have cascading effects on other parts of email processing, leading to functional failures beyond just rejection.

15 Mar 2021 - Free Support Forum - aspose.com

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