How to properly configure DNS & SSL for a mail server?

I have used the following video tutorial in order to create an email server:
https://www.youtube.com/watch?v=9zP7qooM4pY
There's also an accompanying github page: https://github.com/lukesmithxyz/emailwiz

What I've done:
I have registered my domain at namecheap.com and set it to use Linode's DNS servers.
I have used certbot to make a valid SSL certificate.
I have added 2 A records. domain.xyz and mail.domain.xyz
I have enabled DMARC & DKIM & SPF (added TXT records)
I have installed Postfix & dovecot in order to send and recieve mails
Along with some other libraries (in the github script)

The problem arises when I log in on ThunderBird

1. It defaults to use IMAP on 143. Ok I change it to 993.
2. Everything is valid, except it tells me "the SSL certificate belongs to a different site than domain.xyz" with an option to edit. I proceed to edit the location to mail.domain.xyz and then it says "OK all good"
3. The alert pops up 6-7 times. I edit them all to mail.domain.xyz.
4. I am logged in, and I access all my mail folders (Inbox, Sent, Drafts & Junk)
5. I quickly learn that I can recieve mails, but not send them anywhere (not even to myself) Not even sure if this problem is related to the first.

Whoever is reading this, I would be grateful for any ideas on how to trouble-shoot, as I've tried all I could think of.

to be clear "domain" is just a placeholder. I use my own domain i bought.