SSL Error on Mailgun for Mailwizz API URL


I'm getting connection error on Mailgun if I try to use SSL on my mailwizz by generating HTTPS system urls inside MailWizz.
Now I'm using only non SSL but still I'm getting the error on Mailgun. I think it's happening for all SMTP services.

Retry limit reached. Dropped: postmaster@domain.com Could not connect to remote server: [<twisted.python.failure.Failure OpenSSL.SSL.Error: [('SSL routines', 'SSL3_GET_SERVER_CERTIFICATE', 'certificate verify failed')]>]

On WebHook of Mailgun I set the url as "" it was set to ""
The SSL I'm using from Letsencrypt and installed by Certbot on apache server.

How can I solve this issue?
Anyone experiencing the same?

Thanks for your answer.

Seems like it's related to my SSL stuffs. Since I changed to HTTP from HTTPS from "System URL" section and changed the API URL from https to http on Mailgun bounces and other stuffs are showing on Mailwizz just fine no error on Mailgun also.

I just checked with the Letsencrypt SSL using cPanel which is working totally fine. When I'm testing Webhook on Mailgun no error showing but from my production server it's showing error while testing the webhook.

I'm currently using Mailwizz version 1.4.3
I think problem with the SSL on my server. Letsencrypt SSL using CertBot maybe creating issue.
Do you have any idea? @twisted1919