Some more in depth information: My domain is registered through Google Domains. It's for our church. I have setup a virtual site on my COS web server for the domain, since it's a different domain than the primary domain of my COS server. I have a CNAME record for it in my Clear Center account's DNS records, so my Google domain name is a subdomain on my COS server. I also have an A record for it in my Google Domains DNS records that points the domain to my COS server's IP address, which is a static IP through sbcglobal. I can access the website just fine. I don't use the SMTP server on my COS box since I'm not using it as a mail server. The virtual site is a Joomla website and it uses the PHP mailer in Joomla. I have Joomla extensions that send notices to registered users on the website. It sends mail just fine, but mail is marked as spam in Google. I can't have this or my users won't get the notifications I'm sending them.

Based on your advice, I've setup an SPF record in my Google domain DNS settings and also in my Clear Center account. The Google domain DNS setting looks like:

@ for the name, SPF for the type, 1h for the TTL, and "v=spf1 a ip4:99.32.54.26 ~all" for the data.

The DNS record in my Clear Center account looks like: the church's domain for the subdomain, the primary domain of my COS box for the domain, and then "v=spf1 a ip4:99.32.54.26 ~all" for the data.

I did a reverse lookup on my static IP (using mxtoolbox.com) and it shows my static ip.uvs.toldoh.sbcglobal.net. It's also not blacklisted. I do get an SMTP banner mismatch, I think because of this, when using mstoolbox.com to test the email server (i.e. my COS SMTP server). I would suspect most people have this issue since they're not their own ISP. Joomla is configured to use the PHP mailer rather than the SMTP server, so I don't know if this is an issue.