Here is a strange one.
Mac OSX Snow Leopard Server.
Apple Mail app on clients.
Desktops, iPhone, iPad can send and reveive email from outside the network.
MacBooks cannot send and reveive email from outside the network.
All thoughts very welcome, thank you.
James R for GeoffC
All of them, Mac OS X (10.6.4), Some PC's too, forgive me please!...
From the errant clients: use Connection Doctor (Mail.app > Window > Connection Doctor) to test the access.
From the server: Review the mail server log activity around the time of the failures (SMTP server log, POP/IMAP server log, and — if nothing is showing up there — your gateway and any local or gateway firewall logs), perform the usual SMTP/ESMTP server and POP/IMAP server settings comparisons, get off of port TCP 25 for submitting mail from clients, and stay out of 192.168.0.0/24 and 192.168.1.0/24 if any VPNs are in use.
More advanced: Test a connection to port 25 or whatever you're using as a submission port from the remote network, too. You can use telnet or the openssl s_client to connect into the target port (cleartext or SSL/TLS) and verify connectivity. (Though Connection Doctor will usually tell you about this.)
Once you figure out one of the misbehaving Mail.app clients, you'll probably have the rest sorted out, too.
As MrHoffman says, check your mail server logs
Has there been any network changes recently ?
Dns mx changes
Require user authentication to send
Mail not sending for ONLY MacBooks
