I have one of my customers who has sent me a message stating that they are unable to use the "+" sign in his email address
as allowed in the RFC3696 specifcation.

Here is what he wrote>>
The plus (+) character is not allowed in the store. It should be allowed as can be seen in RFC3696. It is used to allow sub-account local delivery options. I personally use it to track important emails of specific types.
I told him that I would look into it to see if there is a fix.