SMTP 552 4.3.1 Session size exceeds fixed maximum session size

Any file attachments will grow by a significant percentage when encoded into base64, so make sure there's at least 30% head room between file size for all attachments and max session size.

wikipedia on base64:

Thus, the actual length of MIME-compliant Base64-encoded binary data is usually about 137% of the original data length

It also Might be worthwhile running wireshark on the server or client to check the content length exactly and ensure that it's actually doing what you told it to do and disconnecting

How many emails are you sending in a single session? Do you disconnect and reconnect for each mail? I've heard this happens if you have too many messages in one connection.

What are the email size limits on the mail server set to? Quota sizes? etc.?

And what exactly are the error messages on the logs of the Exchange server saying, if anything?