It may seem simple/reductive to state out loud, but the custom email address only works if it actually gets delivered to the site.
So you can’t just put anything there, the address must get delivered to the site for it to stand a chance of working.
There’s no way for Discourse to do any verification of that address to ensure that happens, so the onus is on the admin to ensure that happens.
On our hosting I recommend leveraging a few addresses that we’ve prearranged to get delivered to your site:
{ANYTHING}@{YOUR PREFIX}.discoursemail.com{ANYTHING}@{your.site.hostname}