Very small configuration. ISA 2004 running on Windows 2003 as a firewall/proxy for various clients in a small office. Clients use POP3 & external SMTP servers for email.
System's been working for many months with no problems.
Suddenly the system seems to be blocking *large* smtp emails, over 1k or so. This happens for all clients (both Windows & Mac) and any email (pop3/smtp) client - Outlook, Entourage, Thunderbird, OWA. Receiving via POP3 is fine regardless of size, as is web access (streaming radio's fine!).
This means that if you test a client by telnetting to an SMTP server (on port 25, etc.), the email goes OK. Similarly it works if you use the 'test' facility in Outlook (or any other email client). If you send a small email from any email client, it generally works. However once you send a larger email then it hangs halfway through the send.
I've configured OWA on the ISA server to test this and can send emails of any size. I then configured OWA on a client behind ISA and sent a small email - fine. Sending a larger email meant the sending progress bar stops at about 5%. It eventually times out.
Configuring ISA to block SMTP stopped all access as expected (e.g. telnet stopped). Unconfiguring that block enabled the telnet test, and again, the larger emails are blocked.
There are no problems with bandwidth or other access to the web. The server's been rebooted a couple of times and the logs don't seem to show anything particulaly interesting.
I'm really at my wits end with this one! It must be something really dumb going on here.