I thought these addresses are only checked for in the from fiel not the to?

No, to the best of my knowledge it applies to any addresses in the mail. In any case this is how I prevent these kind mails from popping up in my mailbox.

@subven said in How can I block email addresses in catch all?:

via a mail sieve filter

That could work as well, but another mailbox is not necessary. You can just filter on the recipient address.

You could create an actual inbox for the address you want to block and delete or reject all incoming mails via a mail sieve filter.

AKA the /dev/null mailbox

If an address is now leaked (cloudronforum@cloudron.com), can I prevent all emails sent to this address from being forwarded?

Collecting mail via catchall is not common but I have a pretty messed up solution to you problem ^^ You could create an actual inbox for the address you want to block and delete or reject all incoming mails via a mail sieve filter. To make it easier, every time you want to "block" another incoming address, you could make it an alias of said "special" mailbox.

@userino said in How can I block email addresses in catch all?:

Is there such a black list?

yes, its explained at https://docs.cloudron.io/email/#address-blocklist

I thought these addresses are only checked for in the from fiel not the to?

Is there such a black list?

yes, its explained at https://docs.cloudron.io/email/#address-blocklist