There are two parts when it comes to dealing with mails:
- Mail transfer
- Mail deliver
On a very simple level,
- Postfix (MTA, Mail Transfer Agent) transfers mails from your server to other servers, on SMTP Protocol (Simple Mail Transfer Protocol).
- Dovecot (MDA, Mail Deliver Agent) delivers mails from your server to Thunderbird (MUA, Mail User Agent), on IMAP (Internet Message Access Protocol) or POP3 Protocol (Post Office Protocol 3).
Reference:
- https://en.wikipedia.org/wiki/Email_client
- https://en.wikipedia.org/wiki/Mail_delivery_agent
- https://en.wikipedia.org/wiki/Message_transfer_agent
- https://serverfault.com/questions/590209/do-i-need-smtp-when-using-postfix-dovecot/590210#590210
