Scenario: You try to send a message with an attachment to a recipient but you receive a bounce/undeliverable message that shows this error.
Remote Server returned ‘550 5.3.4 SMTPSEND.OverAdvertisedSize; message size exceeds fixed maximum size’
When sending the message, although the message size is close to the max message size set by Exchange, it still falls under the max message size. For Example: The maximum message size that your email servers will allow is 35MB, and the message you want to send has a size of 30MB.
Cause for the bounce: There is a message size conversion when passing off between different message relays. Sometimes this conversion can be up to a 30% increase of the original message size. For example, when the message is received in Exchange it shows 30MB. When the message is passing through the send connector, a size conversion takes place on the message and when Exchange attempts to send it through the send connector to the next message relay, it reads 38MB. This 38MB exceeds the 35MB limit set on the next message relay and it causes a bounce/undeliverable message.
You can see the size conversion in the TotalBytes field by running a command similar to the following:
get-tranpsortserver | Get-messagetrackinglog -MessageSubject “Big Attachment” -start 1/28/2015 | Sort TimeStamp | Select ServerHostname, Source , EventID, TotalBytes
*In my experience, the type of message attachment influences the size conversion for the message.
The easy solution is to just zip the attachment and make it smaller and then attempt to pass it through. Else you need to increase your maximum message size limits on your message relays to account for the increase caused by the conversion. For Example, if you truly want to allow a 35MB attachment and account for a 30% message size increase, your maximum message size needs to be 45.5MB.