"own domain email address" - assuming this means your own domain name and Mail MX records with hosting company or your own nameservers. Have you setup up DKIM/DMARC on your DNS TXT records?
Google have upped the requirement for emails to Gmail and MS to resolve spoofing of email addresses.
Sender Policy Framework (SPF): Specifies the servers and domains that are authorized to send email on behalf of your organization.
DomainKeys Identified Mail (DKIM): Adds a digital signature to every outgoing message, which lets receiving servers verify the message actually came from your organization.
Domain-based Message Authentication, Reporting, and Conformance (DMARC): Lets you tell receiving servers what to do with outgoing messages from your organization that don’t pass SPF or DKIM.
Google have explained it here:
https://support.google.com/a/answer/2466580?hl=en
DMARC in more detail here:
https://en.wikipedia.org/wiki/DMARC
If you put into modem mode, you'll get a new 'dynamic' IP address. If its still fails, then check the above. It probably more simplistic than above, but its good to be aware of changes happening in industry over email sending...