Is this a new page you are having this issue on, or is it an old page created with UE3 that you are updating to UE 4?
UE 4 uses a different path to the helper files. the helper files are now placed in webassist/mail/PEAR_Mail_using_SMTP_PHP.php
it may be that the require once line to include the old file:
is still left on the page.
the PEAR_Mail_using_SMTP_PHP.php file from UE4 does not use the stripfortext function, instead it uses the WA_StripTags function.