After trying multiple approaches, I finally got it working. It turns out the issue wasn’t related to certificates as I initially thought. Instead, the problem seems to be on my hosting provider’s side, where they might be blocking port 465.
This was unexpected, especially since my OpenMediaVault (OMV) setup, using the exact same configuration, works perfectly and sends emails for security notifications. After further research, I came across a post on the forum: SMTP timeout, even though details are correct. On a whim, I decided to try using port 25, and it worked—emails are now being sent successfully.
Hopefully, this helps someone else facing a similar issue in the future!