Attached example output after patch is applied. Now User4 has full validity like
expected, and the debug output shows a match for User4's email address (NOTE:
the debug output has 'YES' for no match and 'NO' for successful match)

D406: 944_example.patch

Attached example patch prevents escaping normal lowercase letters.

Note that this isn't a general solution, though it does solve the issue for me.
For example, some email addresses have numbers (I don't know if having backslash
before numbers is an issue like it is for letters)

Attached example are the following setup:

user1 tsign user2 with full trust, depth 1, domain="customer.com". User2 signs
user3 through user5 (regular signatures). User4 is at customer.com, users 3 and
5 are at example.com.