A couple of years ago I moved from a std Thunderbird mbox to using
fetchmail/postfix/dovecot when mbox became corrupted. Apparently there
is a size limit.

Not being really familiar with these software, I did a lot of reading
and received a lot of help to get my system setup.

Fetchmail gets mail from my email host and gmail.
I use maildir format.
I never really got the gist of Postfix. As I understand it, it receives
the incoming mail and puts it in the folder. That is as far as I used
it. Dovecot then served it up to Thunderbird. When I send email it goes
straight out from Thunderbird.

Recently I have been doing some work that requires me to keep more
strict control over my sending of mail. I understand that I can send
email back through dovecot to be stored in normal sent folder in maildir
along with my trash and drafts. And from there Postfix can pickup the
mail and relay it on through either my email host or gmail.

(as a side: I want to set up my gmail to go through gmail and my mail to
go through my host. Thats my goal at least.)

As I have been reading lately, I learned that Dovecot can also send mail
through Deliver.

you seem to have a pretty good grasp on postfix so maybe you can
speak to this whole thing.

One last thing: 587 Im not told is the port to send to with TLS.
Im using TLS with gmail too.