There seems to be some problem with your activation process.
After filling in the activation form (I tried twice) and receiving the message, that the code has been sent to my email address I did not receive your email (no, it didn’t end up in my spam folder).
Short explanation of your problem:
We got an “Undelivered Mail Returned to Sender” message while trying to automatically send you the code. Probably your e-mail provider is using an AI-based filtering algorithm that blocks certain e-mail addresses. Those tools typically work well for filtering out spam but their efficiency is not 100%. Unfortunately, unless they update their database or provide exact specifications on how to pass their system, there is not much we can do from our side.