It was an automatic response from mailer-daemon@secureserver.net saying 'recipient address rejected: access denied'. I copy-pasted the email address and there were no typos, so I'd assume that it's an issue on their end. I sent the email to Itch.io support instead because I thought that maybe they'd be able to help.
Viewing post in No response to Support Request
This sounds like low level blacklisting of servers. I doubt very much that itch support can help you there.
Either you have your mail account on secureserver or the recipient has. And one of those servers denies the other. Maybe because it is malformed mail, or because from perspective of one, the other is on a blacklist. From what I remember, a mail server can land on such a list, if it is not properly configured and thus allows to be misused for spam sending.
Or do you suspect the mail adress you were sending to is outdated? If it is something specific to your country, maybe you can google the current contact info.
But you should not have gotten an access denied, if the mail adress merely does not exist (anymore).
Unfortunately I don't know much about email servers and I don't know how I would be able to tell what the reason is, sorry.
The email I originally contacted was taxclientsupport-itch@taxidentity.com, which is the email listed in the tax interview FAQ, so I figured that it would've been updated if the address had changed.
For clarification, when I say that I contacted Itch support I mean that I then directed my original question to them instead, not that I asked for help regarding the bounced email. I tried looking but it's not a question that I can search up.
After looking at their site, I have three guesses.
Your mail server is blocked by their mail server, because it is on a blacklist.
It is a temporary issue.
The mail adress is outdated.
And furthermore, I think itch did not answer you because they do not do this type of tax stuff on that support mail.
You might contact support about not being able to send to that mail adress they provided, so they verify if it is up to date and maybe contact the company about those issues if it is up to date yet unreachable.
You might also contact the company, they have a toll free number, if it is urgent. https://taxidentity.com/content/about