mac mail not saving new mail account server settings

hi can anyone help with the following

my mac mail will not save changers i make to incoming mail server. currently the incoming mail server is example (pop.mailserver.com) im trying to change it to example (mail.mailserver.com) this works and the emails start working but when i quit mac mail application and then re-open it goes back to original settings of example (pop.mailserver.com) MAC OSX LION 10.7.5



can anyone help?