I've been running a CentOS/Dovecot/Postfix box as an email server for a while with no complications.
A few weeks ago users stopped being able to send emails through external clients. They can read an access their emails from Outlook/Evolution/Thunderbird clients but they cannot send any email messages.
The error message is very ambiguous to me. It basically says that the client cannot connect to the server.
I haven't changed any configuration files and the maillog is not recording these failed attempts.
I am a rookie at doing this kind of work and I am stumped.
Any help at pointing out any possible causes or any logs that I am missing or any resources that I could study would be greatly appreciated.
Edit: I can send emails through a local web based email client like roundcube or squirrel.
As Matt Simmons suggested I tried telneting the server. I was able to connect from the inside but not from the outside. I then decided to change the port and was able to connect from the outside and send emails from email accounts in my domain from outside clients.
However, now I cannot receive emails from accounts that live outside my server. Please help. I fixed small problem and created a big one.