[FromLine]

Originally Posted by n5bb

However, your image indicates that your Fastmail account is at the fastmail.com domain. Please carefully read my earlier messages and note that the Sieve script does not use any portion of your alias. The two target (Deliver to) addresses entered for your alias are targets at your Fastmail account login address, and the Sieve script contains your Fastmail account main (login) address with +reject.

Now this is my code:

if allof (
  header :contains ["From", "X-Mail-from", "Return-Path"] "gmail.com",
  header :contains ["X-Resolved-to"] ["trueaccountname+reject@fastmail.com"] 
) {
  reject "Please don't use this address!";
  stop;
}
elsif header :contains ["X-Resolved-to"] ["trueaccountname+reject@fastmail.com"] {
  discard;
  stop;
}

I am reading your messages carefully. I'm trying to setup the specific steps, but it's still not working. I just tested this, and nothing is being returned the sender at "gmail.com".