We’ve received a number of reports from Verizon DSL customers that they’ve suddenly lost the ability to send mail. Verizon started blocking port 25 (the SMTP port, used to send email) for DSL and FIOS customers in some areas in 2009, and evidently they’ve now made this system-wide. There’s a Verizon writeup at:
The solution is to change the settings in your email software to use port 587 instead of port 25 for SMTP. If you continue to have problems and can’t send mail, open a support ticket at http://info.his.com/support/support.his.com.html or, if you’re using a Verizon DSL or FIOS connection, call Verizon.
Verizon is doing this to help stop the flood of spam coming from virus-infected customer PCs. A number of other residential internet providers (Cox, Roadrunner, Comcast, BellSouth, Earthlink, NetZero) are doing this as well.