It's because in the mail you want to send to user, you have invalid character in the header whose value exceeds 127.

Quote:

  The RFC 2822 standard specifies rules for forming internet messages.

  It does not allow the use of characters with codes above 127 to be used

  directly (non-encoded) in mail header (it also prohibits NUL and bare CR).

See here for more information:

https://mail.gna.org/public/savane-dev/2005-02/msg00182.html

Regards!^_^
Lostlander.