Hello,
On some occasions I have had problems with sending emails, this problem stems from how the mails are sent, the algorithm is not layers to detect whether a mail server supports HTML format, missing headers of the message in the mail function php, to solve the problem I recommend downloading the libreri phpmailer http://phpmailer.codeworxtech.com/, this would solve his problem and is very complete.
This library can be sent via HTTP and POP3 if tienesalguna doubt my contact is:
[email protected].