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Or do you suspect the mail adress you were sending to is outdated?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).