I tried: ...

Exactly what code did you try? On exactly what data? Please see Short, Self-Contained, Correct Example.

( $sender ) = split (/\s(sender=(.*)@(.*),)/);

This doesn't look like a sensible split invocation. Have you read and understood the split documentation?

client_address=185.15.51.41, EMPTYFROMHERE

I don't understand where this extracted substring comes from; the substring  EMPTYFROMHERE appears nowhere in the OPed data. Again, Short, Self-Contained, Correct Example. Please help us to help you.

Online Regex tools tell me that all is great.

Which online regex tools? How does their advice apply to the operation of split?