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Ubuntu - Email - Install Dovecot
Dovecot is an IMAP server.
It takes responsibility for connecting your email client (Thunderbird, etc.) to your mail box.
Dovecot is configured to provide authentication (username and password) support to Postfix that is used when an authorized user goes to send email via Postfix.
NOTE: Your email client will connect to Dovecot using a protocol called IMAP (or POP).
Dovecot provides access to your mailboxes and when you are reading your email you are using Dovecot.
When you to go send mail from your email client, it connects directly to Postfix using a protocol called SMTP. Dovecot is not needed here.
However, before you can be allowed to access your email you need to prove that you are authorized to access it. This is referred to as authentication. To support this, Dovecot provides support for authentication.
In the earlier simpler days of the Internet before Spam, authentication was not required to send email. However these days you also need to authenticate yourself to prove that you are authorized to send mail. Sending mail is handled by Postfix, but rather than building authentication into Postfix as well, Postfix is instead configured to use Dovecot for authentication when sending email.