I have a Windows 2003 SBS Server running Exchange 2003 SP2.
It runs GFI MailEssentials for Spam, and Trend Message Security for A/V.
Recently mail from some outbound senders does not deliver to internal users. I would guess 80-90% does, but others dont.
Manually telnetting to port 25 reveals a banner and a standard SMTP session. It goes through the HELO or EHLO, MAIL FROM, but when I type RCPT TO:firstname.lastname@example.org it hangs and does nothing.
I ran SMTPDIAG from the Microsoft site and same problem. It goes through the DNS lookups no problem but hangs on the RCPT TO Command.
I have turned off the GFI and Trend Services completely and see no change. Using HELO instead of EHLO doesn't seem to matter.