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The mail adress is outdated.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