AKA anything that isn't @verizon.net (ie @hotmail.com, @juno.com)
Verizon is blocking port 25, please check: http://www22.verizon.com/ResidentialHelp/HighSpeed/General+Support/Top+Questions/QuestionsOne/124274...
Reason: Most common email viruses are sent using port 25 to infect computers. Often times the user never knows their computer has been infected. In order to protect our customers, Verizon has turned off the ability to send email using port 25 for all users other than those using a @verizon.net email address.
They give some step by step instructions for anyone who are using a third party email here: http://www22.verizon.com/ResidentialHelp/HighSpeed/Email/Setup+And+Use/QuestionsOne/124333.htm
Just an FYI if your non Verizon email suddenly stops working in outlook, macmail etc. Simply change the outgoing smtp port from 25 to 587. You won't notice any difference.
Just thought I'd give everyone a heads up if you didn't know about this.