ctype was was added in PHPMailer 5.
UBBT 7.5.x and older used PHPMailer 2 and has several known security issues which are corrected in newer versions of PHPMailer.
UBBT 7.6 and 7.7 use PHPMailer 5.
The ctype extension provides a set of functions that are used to verify whether the characters in a string are of the correct type. The extension is enabled by default if you’re running PHP 4.2 or above. The ctype functions check a string value to see if the characters are within a given range or that every character is of the appropriate type. For example, you can use these functions to see if a string contains only uppercase characters, or if it’s numeric, or if it consists of hex characters, or one of the other dozen or so options available.
ctype functions are always preferred over regular expressions (regex), and even to some equivalent "str_*" and "is_*" functions. This is because of the fact that ctype uses a native C library and thus processes significantly faster.
further reading here:
https://www.sitepoint.com/an-introduction-to-ctype-functions/
