PostFix keeps asking for username and password

Hello,

I have the following problem: PostFix keeps asking for username and password

This is what I did:

I have a freshly installed VPS with CentOS 7.1

I installed perl on it (yum install perl) and then I installed Virtualmin GPL with the install.sh script (so very first thing I did). I configured virtualmin to use hashed passwords instead of plain-text.

Then I made a backup of my virtual servers with the Virtualmin console of all my configured domains on my old server and imported them on my new server. All successful!

Now, I can't send e-mail (using MS Outlook 2013)... PostFix keeps asking for username and password (IMAP is working great, my mails are being downloaded to the mail client).

I have no idea on how to fix this! I tried adding this line to the Postfix main.cf: smtpd_sasl_type = dovecot

But then sending an e-mail fails without even asking for a username and password.

I use TLS and SMTP is set to use port 587 in the Outlook mail client.

Thanks for any help on how to fix this! I honestly have no clue.