Need someone to fix email connection, work out correct ip6 config and get lets encrypt working on server.

I had to move to a new VPS I changed from Debian 8 to Debian 9 and have been having some issues. I imported a site from the old server and everything works except I can’t connect to SMTP via thunderbird. This is a problem for new sites as well so not just the imported one. I also use lets encrypt but I can only get it to work with IP4 if a I have the site set use ip6 as well I run into problems.

Can some one help?