Its possible to tell your mta (like postfix) to use another mta for all mails, or only some domains etc, so using a third party to play the internet facing service then getting the mails by fetchmail, storing them in a dovecot server is easy. on the sending part you could use your standard email client (i.e. thunderbird on pc or k9-mail on smartphone) to send it to your postfix instance that also sits on the server hosting your dovecot service. the mta there takes the mail and delivers it by rules which could just be using the mta of your freemailer using username/password of your account for all outgoing emails. i am doing this but the “external” mail system are my servers as well, i just don’t want emails to stay too long on VMs in the datacenter where i have no access to the physical disks in case something goes wrong.
a raspberry pi is sufficient for such a aetup (i am using a pi4 currently but for emails only i’ld say a 3 or older would do too), adding a disk via usb makes storage huge and cheap then, i use two usb ssd’s in a raid1 for storage… that server could be only accessible through vpn if you whish, depending on your skills and needs (i mainly use ssl client certificates that are supported by k9mail and thunderbird so it fits seamless to be connected through a haproxy that authenticates these before proxying the plain connection to the pi) clients like thunderbird can offline-store all emails (configure download-or-not per imap folder) making searches easy and quick while my k9 client can search locally or on the server if needed.
maybe adjust maximum mail size of your own mta to exactly match (or slightly less) that of the freemailer you use to prevent surprises of big but later then unsent emails.
its possible to have a nextcloud instance on that same pi that acts as an email web mailer just in case of (i really dont need it, but i’ve set this up anyway). nextcloud is also great for syncing/backup files pictures, contacts notes todo lists and calendar of your phone (where i use davx5 opentasks and foldersync for). there are other webmailers available but installing /using nextcloud is not a too bad idea either ;-)
i suggest also setting up some automatic offsite backup with snapshots of that pi then to cover emails and the setup and its configs ;-)
you’re welcome.
what i’ld suggest… a general rule that i like to always follow is to use a test system for everything new. but that does not need to be a full separate system every time.
lets say you have your mailbox and want to try getting new mails from it using fetchmail. first you can use uidl mechanisms to only fefch every mail once and besides that leave them all on the server, but i like it a bit more secure: create a second email adress/account at your mail providers service only for testing. thus you can do whatever you like to to test the mechanisms only without even touching your real inbox (maybe even fill it up with large emails and look how the system reacts, i once had an email account with a cheap provider that deadlocked the inboxes when full…). then when everything is as you want it, switch the account and password (or create another config file for fetchmail) and your’re done. every change (not only fetchmail things) could go tested this way before going live with the changes. filtering could be done with procmail for example, but when the mda that is called by procmail somehow exits with success when the email really isn’t delivered, then the email might get lost forever depending on the settings of course. so fiddling with new stuff always carries the risk of not fiddling correctly ;-)
have fun !