I oftentimes test my email campaigns at this service prior to sending: http://www.mail-tester.com/
It’s one of several I use to confirm that all my settings and spf, dkim and dmarc are working properly.
All of the other services report that my dkim passes but the one listed above always fails. I contacted the operator and they said that they use more stringent tests.
I then contacted the excellent support people at cpanel who are responsible for the software that generates dkim keys on my server. They took a look and suggested I use the smpt send instead of phpmailer and guess what - I passed the test at http://www.mail-tester.com/
I know that phplist suggest the use of phpmailer but this seems - at least in my configurations - contrary to passing all the test necessary to not get tagged as spam.