Ok Till, thanks for your reply.
I just tested a secured connection on Outlook with TLS, and it seems works fine. So, I don't understand why I had these errors in log files...I think it should works now.
Just to understand, I used "courier" before "dovecot" (by following your tutorial on ubuntu). Courier used SSL for secured connection instead of TLS with Dovecot right?
I'm not very fine with mail servers and I want to check if all is good.
So, I have another question. I often have this kind of messages in warn.log:
postfix/smtpd: warning: 18.104.22.168: hostname 22.214.171.124.pldt.net verification failed: Name or service not known
What does it means? Does the email sent to the address can fail?
Many thanks to both of you :-)