@dirtycommo the major thing is having outgoing email coming from a trusted ip-address range. I have had many problems running on a random vps, because of other tenants sending spam. Sending to Gmail, outlook.com etc has been a big problem.

Now I have finally been able to send email via an ip address originating at my employer, and those issues have gone away.

You will also need to implement every anti spam feature out there, like dmarc, SPF and dkim. That part is fairly easy though, and you can definitely do it.