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I keep getting this error message when sending some emails from my gmail account

Dorothy Adams 5 Reputation points
2025-11-21T19:13:37.07+00:00

Technical details 

MessageDeliveryFailedException: Could not deliver the message [len=70, data=000000002343918A5889644E9E0C79FB7EEEEB310700CE6F36BF74141444A696867FA00AB37400000000010F0000CE6F36BF74141444A696867FA00AB374000127867C970000] sent at 11/21/2025 4:59:46 AM.
Failure code: c355
Microsoft 365 and Office | Other
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  1. AI answer

    2025-11-21T19:14:16.2833333+00:00

    The error message you are encountering, specifically MessageDeliveryFailedException, indicates that there was a failure in delivering the email. The failure code c355 suggests that there may be issues related to the message format or the recipient's email server rejecting the message.

    Common causes for email delivery failures include:

    • Incorrect recipient email address.
    • Issues with the email server settings or configurations.
    • Problems with the content of the email, such as attachments that are not formatted correctly.

    To troubleshoot this issue, you can:

    1. Verify that the recipient's email address is correct.
    2. Check if there are any restrictions on the recipient's email server that might be blocking your message.
    3. Ensure that the content of your email, including any attachments, adheres to standard formatting guidelines.
    4. If the issue persists, consider contacting your email service provider for further assistance.

    Unfortunately, without more specific details on the context of the error, it is challenging to provide a definitive solution.

    References:

    AI-generated content may be incorrect. Read our transparency notes for more information.

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  2. Anonymous
    2025-11-21T20:07:04.38+00:00

    Hi Dorothy Adams, I can imagine how frustrating this must be for you dealing with a persistent error like c355 when you just want your emails to go through can really disrupt your workflow.

    To resolve the c355 error enable detailed SMTP logging to capture Gmail actual response codes validate message encoding for proper MIME compliance, temporarily disable Outlook add-ins and antivirus to rule out stream modification, re-authenticate using refreshed OAuth tokens or an App Password if 2FA is enabled, and finally test sending a minimal RFC-compliant email via raw SMTP to confirm whether the issue lies in the client’s serialization layer.

    When you checked the SMTP logs did you see any Gmail response codes like 550 5.7.x or 421 4.7.x, and if so what were they?

    Best regards

    Lucky Charm

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