You need to check with your e-mail provider, they will tell you the alternative ports. Mine uses 2525.
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With Roadrunner mail the outgoing server settings vary depending on wether or not you are processing the mail through your Roadrunner suppied modem or not. If your iPhone is connected through your Roadrunner modem (i.e. you are connected to your home wifi network) the SMTP server is set to port 25. If you are outside of your home network the SMTP server is connected through port 587. You can create an additional SMTP server and name it Roadrunner:587 (the colon will tell the setup to use port 587) and enable both servers. That way if you are at home and connected through your wireless lan (much faster than 3G) it will use the port 25 enables server and if you are on the road it will use the port 587 server.
Hope this helps....Enjoy
IPhone Road Runner Issues - FixYa