In the previous commit, I removed all calls to the deprecated
utf8_encode() that seemed safe to remove. In this commit, I'm replacing
the calls to this function that I'm not sure if are same to remove or
not with mb_convert_encoding().
mb_convert_encoding() requires the extension mbstring to be enabled. It
should be enabled on most PHP install but not all. We are already using
mbstring functions in our code base and we provide a polyfill for PHP
installs where the extension is not enabled
(62bb75ed91/mailpoet/prefixer/composer.json (L25)).
So it should be safe to use it.
[MAILPOET-4865]
Instead of using second mailer with a different configuration we just send from the PHPMailer
as it was configured in wp_mail function.
[MAILPOET-4254]
