automated emails from my bespoke php website are not getting into my users' inbox

i have a website which regularly sends out emails via cron command to all my users (around 40 email addresses). some of my users are receiving the email, some are not (and it is also not in their spam folder either). i contacted my host and they said it can be due to the content of the email said that it might be in the form we're sending and that the mail client (e.g. google/hotmail) are detecting the mail as spam and killing it because it's being sent from a server which is not specifically being allowed to send mail for that domain name (this can be done with an SPF record in the DNS for the domain name). I have attached one of the php files from which an email would be sent. i think there might be bugs in the php files which need fixing so they all need checking over. i don't totally understand this, but I need it sorting out so all my users get the regular emails. if this is fixed, i have some other tasks which i would like done on this website also.