This information is often buried on a support page so I thought I'd put up a list that makes things a little more accessible. Here are the mail servers for the four main ISPs for the NYC area.

Earthlink
POP: pop.earthlink.net
SMTP: smtpauth.earthlink.net (turn authentication on, port 587)
Webmail: webmail.earthlink.net

Cablevision/Optimum Online
POP: mail.optonline.net
SMTP: mail.optonline.net
Webmail: webmail.optonline.net

Time-Warner/Roadrunner
POP: pop-server.nyc.rr.com
SMTP: smtp-server.nyc.rr.com
Webmail: webmail.nyc.rr.com

Verizon
POP: incoming.verizon.net
SMTP: outgoing.verizon.net (turn authentication on)
Webmail: netmail.verizon.net

Most ISPs only support sending mail though their SMTP server if you are connected to their networks. If you are connected to another network you may need to use the SMTP server of the ISP you are connecting through.