james
It is no longer "easy" to host your own sending mail server that will be trusted by other mail servers.
If you are not prepared to do what is necessary to give your mail server a high reputation, then the answer is to send your mail via your ISP's smtp server (configured in server manager Email panel), that way you rely on the reputation of your ISPs mail server (which is usually or should be OK).
If you want to host your own mail server & send directly to other mail servers then you need to ensure your static IP/sending server/domain has a good/high reputation.
If you fix things that make google accept your mail then you are likely to fix things for other recipient ISP's also, but others may have specific requirements you will need to implement, refer partially to the wiki link I gave earier, a lot more has been written in the Forums etc since those words were published (eg DKIM, SPF, DMARC).
From here (& there are more links in the original article):
https://support.google.com/mail/answer/81126#authenticationAuthentication & Identification
Why is it important to authenticate your messages?
Authentication ensures that your messages can be correctly classified. Emails that lack authentication are likely to be rejected or placed in the spam folder, given the high likelihood that they are forged messages used for phishing scams.
In addition, unauthenticated emails with attachments may be outrightly rejected, for security reasons.
To ensure that Gmail can identify you:
Use a consistent IP address to send bulk mail.
Keep valid reverse DNS records for the IP address(es) from which you send mail, pointing to your domain.
Use the same address in the 'From:' header on every bulk mail you send.
We also recommend the following:
Sign messages with DKIM. We do not authenticate messages signed with keys using fewer than 1024 bits.
Publish an SPF record.
Publish a DMARC policy.
Learn more about email authentication.