Whether you set up your e-mail address directly on your iPhone or sync its settings through the iTunes application, the iOS operating system automatically sets the account's outgoing server as the primary SMTP address for the phone. If you are having trouble sending messages from your phone, you should check the primary SMTP server settings. You can make changes to the server address right from the iPhone interface.
Tap the Settings icon on your iPhone's home screen.
Go to the "Mail, Contacts, Calendars" category in the menu.
Tap on the name of the email account that you want to modify.
Click the field labelled SMTP under the Outgoing Mail Server heading.
Tap the entry under the Primary Server heading on the next screen.
Tap the fields you want to edit and use the onscreen keyboard to adjust the SMTP server address, port, user name and password.
Press "Done" to save the changes you've made to the primary SMTP server.