I am having trouble with other servers rejecting emails. The recipients are large enterprise companies (neweggbusiness.com being one of them) so I assume they have strong mail filtering requirements. My SPF record has been in place for a long time but when I checked it with MXToolbox they said it was wrong so I recreated my SPF record and it checks out ok now. I do not have a DKIM or DMARC record setup, the first time I ever heard of it was when I started researching issues why I might be getting blocked.
The NDR report says that google.com checked my SPF record and it passed.
The error message I am getting is < #5.7.1 smtp;550 5.7.1 RESOLVER.RST.AuthRequired; authentication required> and the generating server is the far end, not my own. Other discussions about this error point to the far end that they are blocking anonymous requests but I doubt that it is the other organizations that I am emailing.