I am having trouble finding a solution for this problem. We are running SBS 2003 with Exchange 2003sp2. When outlook or webmail clients send emails with attachments over a size of about 20kb they sit in active/retry until the timeout is reached. The clients then receive an NDR. Emails with no attachments are delivered successfully. Emails with small (<20kb) attachments are delivered successfully.
The interent ip is not on any blacklist, reverse dns is correct, and I can send a basic message with no attachment using telnet commands. I have tried adjusting MTU on NIC to match router as some have suggested but this has not worled. SMTP logging does not give any indication why the connections are being dropped. I have tried different smart hosts with and without authentication to no avail. I have temporarily lifted the allowed attachment size but this has made no difference.
I have tested with smtp connector and without. Any ideas welcomed.