I recently had a domain that I wanted to let expire, and my domain registrar kept sending me emails about it, with no way to turn them off. They suggested changing the email contact to a throwaway address. I couldn’t figure out how to do this in Fastmail so I contacted support.
Thank you for contacting Fastmail! I would suggest setting up an alias that goes to your main inbox.
After creating the new alias that you don’t want email for, go to your Settings > Rules.
Settings:
Users & Aliases > + New Alias
Enter address > Save
Click on +Create Rule.
Change From to To/CC/BCC and enter the alias you don’t want in the text box.
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Press Continue.
Then, click on the Create Rule button on the results that appear. Click on Delete to Trash and save the new rule!
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This should do exactly what you want it to do — automatically delete a message sent to a particular alias you’ve created.
So that’s how to setup a ‘blackhole’ email account for spam catching or for any other purpose.
These steps are obviously specific to Fastmail, but in general terms should work just as well elsewhere.