Revert of "Use PHP's own email validation routines"

This partially reverts commit 4235a08618da1d66b44337867b72f4fdea633dc0
which introduced a regression.

The PHP email validation rejects valid e-mail addresses with the form
user@hostname [1]. The PHPMailer accepts these, and furthermore it makes
sense to rely on the validation routine provided by the library we use
to send our emails.

[1] https://bugs.php.net/bug.php?id=49576