Your message very likely looks like spam and you are getting a false positive. The best would be to check the 'spamminess' of your e-mail using Mail Tester.
Indeed, this free service is literally for testing the 'spamminess' of your e-mails and avoid false positives. You just need to send your message to an e-mail address they will give you. A few seconds later you will get a score and a detailed report of what is ok and what is wrong. That simple! In addition, you get some directions to fix the problems. The best here is to try to get a score of 10/10.