Emails from customers and reply with forward

[freddyvl]

Hi,
Can anyone tell me what configurations must be in place in order to allow customers to send emails to a particular queue and for Agents to reply to any type of ticket(is it Forward ?). Currently the settings I have 3.0.11 help desk and 3.0.5 itsm . have set an email adress in postmaster emails using pop3. This downloads all emails in that particular email account. But when I try to reply to a Customer ticket using forward it gives me an "impossible to send email" error. I have even set sysconfig-> framework ->sendmail-> username and passwd using smtp.