Help with SMTP for Optonline.net

I'm new to iphone and just purchased a 3G phone. It worked great for a couple of weeks sending and received everything great just lovinging my phone. Then just yesterday I attempt to reply to an email and received and error message. Cannot send mail The connection to the outgoing server "mail.optonline.net" failed. Additional outgoing mail servers can be configured in Setting > Mail, contacts and Calendars. Call the store I originally purchased the phone from and they were to busy to answer so now I'm attempting to get the answer and all I find is to go to setting, mail,contact,calendars,click on mail and outgoing mail server SMPT to mail.optonline.net then type my user name and password, and I still get the same message. HELP PLEASE! I love my phone and want to continue loving it.