@john
@alex
SugarPHPMailer;php is part of the application, so any modification to it would be done via pull request, tested and if good, added - it may take them 2 days, or 2 years, there’s little rhyme or reason to their process - except that they’re too busy making money managing SuiteCRM for the English National Health Service to really care too much about getting automatic testing working so they can update the software with Pull Requests in 24 hours not 24 months ! (facepalm)
If you don’t want invest so much of your time and work to develop a pull request, then enjoy getting ignored and face that frustration, the straight forward approach is to just use a free open source control panel (virtualmin) for your Linux SMTP mail server (postfix) which auto generates the key pairs and signs your outgoing mails with DKIM. Such as this one:
https://www.virtualmin.com/documentation/email/dkim