The out going mail server as it was triggered as spam.
Most modern spam filters operate on a point system where points are based on key phrases and message formatting that are known spam triggers or watch words.
To Avoid this being triggered I would advise the below.
Avoid using URL’s and links.
Avoid using prominent images.
Avoid excessive use of capital letters.
Avoid repetitive use of words like “Free!” And “50% off!”.
Try to use simple subject lines.
Avoid using quotation marks, asterisks, dollar signs, exclamation points or or other punctuation.
Including a toll-free number in the subject line.
Avoid using a large font size in message body text.
