If it sends to some users but not others, that is a problem with the smtp server.
you will need to contact the host to have them look into the smtp server error log to determine why the emails are not sent to those addresses.
as for the email being flagged as spam, that can be any number of things, it could be the from address being used, the message title or the message body. if you view the message source, there should be an indication of the spam rating from the spam filtering service that should include details on the why it was flagged.