5.3.4 Header size exceeds fixed maximum size

Describe the bug We upgraded from 2.0.3 to 2.3.0 and had issues with attachments when sending through smtp.office365.com

In the logs we would see the final attachment boundary, and then a vague error.

C: --=-DdAczYjEkl/ABnOTLAA2UQ==--
S: 552 5.3.4 Header size exceeds fixed maximum size
C: RSET
S: 250 2.0.0 Resetting

See the screenshot for a comparison of the 2 versions. It seems that message size, and BDAT options have changed between the versions, which is enough to trigger our issues.

If possible, I’d like to know:

1. Is there a mail server setting we can change to remediate this issue for 2.3.0?
2. Is there setting in MailKit 2.3.0 to use the older style of message?

Screenshots If applicable, add screenshots to help explain your problem.

Server (please complete the following information):

• .NET Core 2.2

[jstedfast]

Confirmed. I just commented out the code in FilteredStream that flushed the source stream in its own implementations of Flush and FlushAsync and sending via office365.com worked fine.

So it seems that there was a bug in MimeKit and also a bug in office365.com’s SMTP server implementation because it should not be freaking out when it receives smaller packets.

[ghost]

@jstedfast Personally, I am not. The solution for me was to disable size and BDAT, so I’m not waiting on the fix to be released. When it is released though, I will update to the latest version of the nuget package, remove the line of code that disables size and BDAT, and try sending that message again that had failed prior. Thank you for all of your guidance, troubleshooting, and fixing of the issue!