We're selling a web application that customers install on-premise or in different cloud environments. The default email setup sends email via a web service we created 15 years ago to one of our servers. The web service on our central server create emails in the IIS SMTP pickup directory for delivery. We have always warned our users that this web service is just for testing and development, but many are using it in production.
A few years ago we started getting delivery problems, so we adviced all our customers using the service to setup SPF. That helped a lot, but lately we have started to have some delivery issues again. I think the main problem now is that we're not using TLS, so Gmail for example has a question mark beside all the email from our server. I have started to look into setting up TLS, but I'm not sure how to do it correctly. If we only sent emails for one domain, it would have been easy, but we're sending emails for around 30 domains.
In tutorials I see they specify a FQDN when configuring TLS setup. But how do we do this when so many domains are sending through our server? Can I just setup a sertificate for mail.mycompany.com, and that will be used for all domains?