[Helmling]

I'm experiencing odd mail behavior with my .Mac account.

I'm trying to access over an open wireless network at work and I'm receiving mail just fine, but it gives me an error trying to access the SMTP server.

I don't get it? Could it be a port setting or anything like that?

[Helmling]

Ok, update...no longer urgent, but I plugged into a wired connection and suddenly it works.

Can anyone explain to me what about a wireless network would prevent an smtp server from working? Weird.

[CatOne]

Whose wireless network was it? Many networks block outgoing SMTP to combat spam. And your ISP's mail server is almost never accessible when you're outside their network -- anonymous remailing is pretty much ALWAYS blocked now.

[Nivag]

Have you tried changing the Port for the outgoing server?

have a look here on how to do it for a .Mac account

.Mac: Issues sending messages from .Mac account in Mail or other email applications

[MRTrauffer]

From my experience, some ISPs use their own SMTP servers for outbound mail.

By default, Mail.app sets the .mac outbound SMTP server as smtp.mac.com, but for my home network connection via Cox Cable, they require that outbound mail be sent via smtp.east.cox.net.

You may need to check out the support page for your ISP.