As I mentioned via PM, I can’t give access to the server due to security concerns. But thanks very much to @pfaffman for your help and for trying to solve this issue.
Let me add more context to this issue: a previous admin installed Discourse with the Mailgun SMTP service but it stopped to work and I don’t have access to that account.
As I said, I am trying to configure it now with Mailersend. I have read this topic [1] and others about STARTTLS here in the forum, but I am not sure about how to implement the changes needed.
I tried this setting below as well but the errror remains
DISCOURSE_SMTP_OPENSSL_VERIFY_MODE: none
If it is related to some recent update, maybe is it better to consider a downgrade then?
[1] Can't send email with certificate issue - #3 by supermathie