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Old 5 Dec 2019, 06:59 AM   #10
Intergalactic Postmaster
Join Date: May 2004
Location: Irving, Texas
Posts: 8,706
Originally Posted by rmmarsh View Post
What I want to accomplish is when a person tries to respond to the email I have sent them, I want the Fastmail Sieve code to reject their reply with a custom message other than "550 No Such User Here"...
If the From address you use at Fastmail when you create the message is a valid Fastmail account or alias, then you won’t get a 550 SMTP error when they reply. The “550 No Such User Here” message is not generated by Fastmail, but as I noted earlier at some other email system. When you send a message to a non-existing address at a Fastmail-owned domain, the sending MTA SMTP server receives the response:
 550 5.1.1 <bad address placed here>: Recipient address rejected: User unknown in virtual mailbox table
The person sending such an email to a non-existing email address will receive a message which is generated by their own email system. It might contain some of the text shown above, or it might be something like the text you quoted.

If you create a From address at Fastmail for only this purpose, you can use a Sieve rule as you described to create a response. But it is a reject message which will contain the initial content as I described in an earlier post.

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