Simple Inbound Email Gateway(s)
Creating a simple mail gateway for migrating off services.
https://xdeb.org/post/2017/12/20/mail-relay-mx-backup-and-spam-filtering-with-postfix/
The above is about as simple as it gets.
Introduces front door spam filtering and even includes archive email "backup" user example.
You will probably want to expand the above example to do backups to separate email accounts based on user as it gets hard to search a billion emails and more importantly you probably only want the user/manager viewing there archived email.
Sending incoming email for a user to another domain (two total).
This is useful for migrations.
Create a map file on inbound mail gateway running postfix:
# echo "jeremy-busk@example.net jeremy-busk@example.io" >> /etc/postfix/sender_bcc
Mail will continue to go to jeremy-busk@example.net while sending duplicate copy to jeremy-busk-example.io.
Create a hash for the map file:
# postmap /etc/postfix/sender_bcc
Configure Postfix to read the hash file and reload the configuration to apply the changes:
# echo "sender_bcc_maps = hash:/etc/postfix/sender_bcc" >> /etc/postfix/main.cf
# service postfix reload
To disable for user just comment out the userline line in
/etc/postfix/sender_bcc &
# service postfix reload
Email will continue to flow to previous domain as it did before it just wouldn't get copied to user.
Syncing email as an option for migrations.
https://imapsync.lamiral.info/
There might be a better tool out there now but this ruled the world for many a year.