Ubuntu Postfix + Dovecot external mail set up: will send but wont receive

This is probably a dovecot problem.

Because if you send an Emails in only goes through postfix. The above error message tells you that postfix can’t connect to dovecot the same goes for thunderbird.

Make sure dovecot ist running and listining on port 143 IMAP

Also for lmtp tarnsport you need something like this in your dovecot config:

service lmtp {
    unix_listener /var/spool/postfix/private/lmtp {
        user = postfix
        group = postfix
        mode = 0660
  }
}