Credentials are “Incorrect” When Logging in to Postfix Mail Server

Recently I wanted to play around with mail servers, so I followed the steps in this article. But when I tried logging in with Mozilla Thunderbird, it gave me this error:

Configuration could not be verified

I tried different accounts and changing passwords, but it hasn't been working. What is going on here?

Also, does anyone know of any mail servers better than this one?

Any help is highly appreciated

Thanks

EDIT:

There was a typo in this file:

/etc/dovecot/conf.d/10-mail.conf

There is this line here:

mail_location = maildir:/path/to/mail/

Which I forgot to include maildir: before the path.