Xtra (Spark) has an issue with their email server, it is refusing mail from many sources, not only affecting the ShowSec system, but also others such as gmail!
I suggest you phone xtra/spark and complain about it.
If you use an xtra email address, then you can expect to not be getting email from us, or from many others also.
Personally I recommend not using xtra or spark for email at all, I have been trying for over a week to get someone there that actually knows how their systems work and that understands their own servers error messages.
ShowSec is not in any blacklists, and we use proper email systems to ensure that our mail is likely to get through, this issue lays squarely with xtra, so complain to them if you don't like them filtering out legitimate email.

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