Something happened server side yesterday and Apple’s Mail.app is no longer able to pass authentication with the outgoing mail.optimum.net SMTP server. What’s strange is that it only affects Mail.app. Sparrow and other mac email clients are unaffected by this.
The other odd part of this is that tech support at Cablevision admitted something went wonky on their end when they did some kind of server upgrade. Normally I’d expect, “change your password” or “reboot your modem” or other canned responses but tech support was forthcoming about the issue. No telling when it will be resolved though. Better be soon since Verizon FioS just came to terms with BeInSport.
If you’re having this problem, see the comment below. Forcing port 587 for outgoing mail does the trick.