I think the cache is not utilized at all. Noticed when inserting print statement inside return cached value block.

Thank you, indeed helpful!

Here is the standard:
https://www.ietf.org/rfc/rfc5322.txt

Have you tried a regex?
/[a-zA-Z0-9.@]/gm

Also, besides letters, numbers, and the two characters you included (. and @).
Alot of other chars are allowed. i.e. (!#$%&'*+-/=?^_`{|}~ etc)