Ubuntu email server help with postfix

[exxos]

Tried countless guides on the net to get a email server working with postfix and dovecot.  Mostly I can get the thing running to the point that mxtoolbox says all is fine on mail test, all the security stuff is there, but after that, it turns into a nightmare. Mostly I have been trying to get it using a SQL database for users and such, though I can never get logged into the email account via thunderbird.. Testing email on the server itself, im not sure works. SQL stuff seems all good, but overall, its driving me nuts. 

Really wondering if anyone knows of a known 100% working guide, or anyone who knows how to set one up which wont charge the earth.

[nowster]

I use exim with dovecot. How big is your database of users?

[exxos]

I only have about 12 mail boxes across 4 domains currently.  I have got postfix and dovecot working, but nobody can send me emails, but I can send them, and send emails to myself in thunderbird, just I can't get emails from people outside of my domain. TB just says uder not found, which is odd, as the account is there as I can send emails to others 

[nowster]

I have about the same across about the same number of domains on exim, with the passwords synchronised for receiving and sending... but have little or no experience of Postfix.

I used to be able to craft a sendmail.cf by hand. (None of this m4 rubbish.)