"552 5.3.4. Header size execeeds fixed maximum size" error message from Exchange

Emails with very long email headers (often due to overly long lists of recipients) are rejected by Exchange with this error message. Unfortunately POPcon can't do much about this and moves these emails into the BADMAIL folder (inside c:\program files (x86)\POPcon).

If you want Exchange to accept such emails you can use these Exchange Management SHELL commands:

First get a list of receive connectors and their maximum header size limits:

get-receiveconnector | format-list -property identity,maxheadersize

On our own server the output looks like this:

Identity : WIN-DKVEC8768RS\Default WIN-DKVEC8768RS
MaxHeaderSize : 64 KB (65,536 bytes)

Identity : WIN-DKVEC8768RS\Client WIN-DKVEC8768RS
MaxHeaderSize : 64 KB (65,536 bytes)

Identity : WIN-DKVEC8768RS\local apps relay
MaxHeaderSize : 64 KB (65,536 bytes)

Now select the receive connector that is active on your own server. Most of the time it will be the "Default YOURSERVERNAME" connector. To change the maximum accepted header size use this command:

set-receiveconnector -identity Default* -MaxHeaderSize 256kb

You'll see the value changed when you do another get-receiveconnector like above again.

You can have POPcon retry the emails that were dropped in the BADMAIL folder by manually moving the *.msg files from the BADMAIL to the PICKUP folder. POPcon will then retry forwarding these emails to Exchange again during the next mail cycle.