If you're using Let's Encrypt certificates with hMailServer, you might want to search your logs for "no shared cipher" messages. If you're getting a lot of these, it may be because certain LetsEncrypt clients have started defaulting to generating Elliptical Curve Certificates (ECC), rather than RSA certs. Apparently Outlook/Hotmail and certain other MTAs don't understand elliptical curve certs.
Whether you have an ECC or RSA cert will depend on the Let's Encrypt client you use to generate the cert and how it is configured. AFAICT the standard certbot client still defaults to RSA certs, but it looks like a number of third-party clients (including some Windows-specific clients) default to ECC. If your client defaults to ECC you'll have to consult the client docs to figure out how to switch to RSA. (I'll update the instructions I posted for the Lego client.)
In my experience, outgoing Outlook MTAs will at least retry without STARTTLS, so the email will eventually go through, albeit unencrypted (although apparently this may not have always been the case, see this thread). However, I have seen some other senders who do NOT retry w/o STARTTLS, so those emails never get delivered.
There are good reasons to default to EC certs for web servers, but for now (and probably the foreseeable future) sticking with RSA is a better choice for email.
Use this forum if you have installed hMailServer and want to ask a question related to a production release of hMailServer. Before posting, please read the troubleshooting guide. A large part of all reported issues are already described in detail here.
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