purtroppo anche io ho questo problema.

Sul manuale di PHP ho letto che il problema potrebbe risiedere nel Message-ID degli Headers:

/**************
"I've noticed some anti-spam email programs also deny delivery to mail that doesn't have a Message-ID. Even though it is legal to do so, many domains will consider the email spam and drop it.

Using sendmail on a win2k server, there is no Message-ID on some test mailings I've done. The ones that are added were there due to the filtering software added it afterwards, at least that's what the spamheaders error code database says.
"
********************/