Hello,

I have installed a mail server on a linode with Centos 5.0. All is good as far as I can send and receive emails. I am using postfix

So far, everybody has been using squirrel, but here I have someone who want to use Mac's Mail app instead. Getting the incoming email was straightforward but I am failing for the setup of the outgoing email

The outgoing server is something like xxx.members.linode.com but how do I know the username and password and maybe the port?

Furthermore, it seems to me that the real problem is that only mails sent from the server can be processed by postfix. Even with the right username and password, it seems that I would not get much further. Is that right?

Could someone shed some lights here and maybe point me to some way to have that person use its Mac's Mail app to send email in a safe fashion.

Thank you

I'm not sure about the details of your setup, but it sounds like you need to configure SASL. If you have squirrelmail running then you probably have either Dovecot or Cyrus running for your IMAP server - either can be used to allow the authentication of SMTP users. I'd also recommend setting up TLS so that you aren't sending your passwords through the internet without encryption.
Lots of howtos can be found here, and hopefully one or more will give you the info you need to get started.

If you are receiving emails, then postfix is definitely processing mail from port 25. If you get SASL working, then it should allow you to send outgoing email as well. The simple approach is to allow connections to the normal SMTP port (25) to authenticate and send out emails, but there are some advantages (and complications) to using a different port (587) for submission.