How do I change the password for my incoming and outgoing mail servers?

I am not able to receive or send email on my imap account.  This is the error message that I get.  

Cannot Send Mail
The user name or password for "patmoell@bellsouth.net" is incorrect.

Is the user name "patmoell@bellsouth.net"?

If so, the passwords must be wrong.  What can I do to change the passwords?  I don't know the current passwords.  

Here's more information about the servers.

Incoming Mail Server

Host Name  imap.mail.att.net
User Name patmoell@bellsouth.net

Outgoing Mail Server

SMTP  smtp.mail.att.net


Other SMTP Servers

smtp.mail.yahoo.com
outbound.att.net

Since I can't receive email, can you answer this another way?  <email removed>

Thanks.