Are you configuring a single IP address or a single host name?  A domain name will not work.

Things that will help avoid suspicion:

Static rather than dynamic IP address.

Encrypt with TLS 1.2 or 1.3

Host name matches Reverse DNS name and both resolve to your static IP.   Changing the Reverse DNS entry requires a SWIP entry created by your ISP.  I think it stands for "Shared Whois for IP".

Not sending spam.

Not sharing a server with someone else who sends spam. 

Hi Douglas Foster 

Thanks for sharing your solution to this!

Hello Jesper, 

You are correct that this is an Enterprise Feature

vs Enterprise

When sending to gmail.com and any other mail servers you will want to make sure that the IP reputation is good and that the DNS Authentication records are set up as well. 

DNS Authentication is used to prove that the email is coming from you and not being spoofed. The main records you will need are SPF, DKIM, rDNS(PTR). You will also want o consider getting a DMARC record added as well. 

The DKIM (DomainKeys Identified Mail) record is the only one that you will need the SmarterMail web interface to get generated. Here is the help on how to get this set up in SmarterMail (Email Signing). 

To get a better understanding of the DNS records needed you can review this article. (Understanding SPF, DKIM, DMARC)

rDNS (PTR) will be set up by your hosting provider unless you are running your own DNS service. If you are running your own this will help you to get his added. (Configure DNS PTR Records)

You can verify your rDNS/PTR record here ->https://mxtoolbox.com/ReverseLookup.aspx

You can verify your SPF record here -> https://mxtoolbox.com/spf.aspx

You can verify your DKIM record here -> https://mxtoolbox.com/dkim.aspx

You can verify your DMARC record here -> https://mxtoolbox.com/DMARCRecordGenerator.aspx

Hope this helps.