Postfix + Dovecot: Create send-only email account

The easiest solution would be to add the user just like any other user, but restrict recipient. This will give an error during RCPT TO regardless whether the user actually exists or not.

For example, if a web page of a company might send some emails using [email protected] as a sender address, while no-one should read that address but only their [email protected] addresses, and the server has mydestination = example.com that would normally deliver to that user, I'd simply add (without removing what there already is in smtpd_recipient_restrictions):

smtpd_recipient_restrictions =
    . . .
    check_recipient_access hash:/etc/postfix/access/denied_recipients,
    . . . 

check_recipient_access type:table

Search the specified access(5) database for the resolved RCPT TO address, domain, parent domains, or localpart@, and execute the corresponding action.

Then add the unallowed addresses there with a friendly and human readable reason for the reject:

[email protected]    REJECT    Visit the web site for correct contact information.
[email protected]   REJECT    Please log in and use the helpdesk contact form.

That should work what ever is the reason you don't want to accept mail for an address. Remember to postmap /etc/postfix/access/denied_recipients as the lookup table is a hash type Berkeley DB.