Hello,
I don't see why the number of character in your email address would affect the Paypal communication.
If you were able to make it work using another account, it might be related to something configured in your first one, like the accepted currencies or something else (hard to tell without an error message).
Paypal support would be best for such question.
Regarding the limit feature in HikaShop starter, I don't see any mention in the documentation page that I gave you.
But you can open a new thread in the HikaShop section of the forum in order to ask the HikaShop support team
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.