A common desire for circumstances like this is for the user to be able to receive email from multiple addresses but also to be able to send from a specific address.
As has been mentioned, you can have the user receive email on multiple accounts by adding additional SMTP addresses.
By default, when the user SENDS mail it will be from whatever email address is their default, without consideration for the account the mail was sent to.
To get around this, at the client system you can add a new IMAP email account and point it to your server. IMAP accounts leave mail on the server, so you won't be adding any excess traffic or files. This will then give your user a new button near the Send button that will let them choose which account to send From.
I suspect that the combination of this and the above advice will satisfy your desire for three mailboxes.
Your user can further enhance this by creating three folders in their mailbox and use rules to direct incoming mail from each address to its respective folder, thus isolating the three addresses.
I hope you find this post helpful.
Regards,
Mark
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