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> I don't have any idea on how to prevent outgoing email to other email services.
I've no idea how to do it with Postfix etc, but I /have/ in the past been able to do this with one specific mail client (unfortunately no longer maintained so I cannot recommend it).
It supported filtering of outbound Mail (also: outbound News, incoming Mail, incoming News, items seen when a folder was opened, and items about to be considered for automatic expiry).
I used outbound filters to make sure that only email addresses of mine which I used for emails (or news posts) to certain places were used when mails / posts were sent to those places. For example, that a mail to a mail-list server could not be sent using an address which I intended only to be used for my bank, and vice versa.
Although usually mails/posts got composed with appropriate sender ids, sometimes I'd accidentally pick the wrong one off a drop-down list of alternatives, or an automatically chosen sender id would be wrong. The latter could happen if eg all replies to items stored in a particular folder would automatically get written using that folder's default sending id, but a mail had incorrectly been placed in the wrong folder.
The outbound filters in essence prevented the mails / news posts concerned from being placed in the outbound mail/news queues and would instead put them in a "Blocked send" folder (and play a sound to alert me that that had happened).
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