We are sometimes asked why emails are being flagged as spam. Mail servers use a disparate variety of methods to combat spam and in most cases we are not in control of the system that is flagging these emails as spam. This is therefore not a question we can answer with authority, but is something the recipient network administrator can answer. However, two common issues have emerged:
1. Message content - your message may contains words or phrases which are spammy.
2. SPF record - mostly affecting mail sent from outside our network. Explained below. The default SPF record we give domains does not allow sending from all IPs, rather it specifies only the A and MX records.
What this means is that unless you've made changes to your DNS by either changing the SPF record or the MX record, mail sent from an external mail server masquerading as mail from a domain on our network has the potential to be flagged as spam by a receiving mail server. If you are finding emails being flagged as spam and are not using our mail server to send, you can try changing the SPF record (currently a TXT record in the DNS) to "v=spf1 +all" without the quotation marks.
This may result in your messages no longer being flagged as spam. Be aware that changing the SPF to +all will result in mail from any server passing SPF, and increases the chance that a third party can pretend to be sending from your domain without being flagged as spam.