Noted. I was under the impression that you mean SSL on port 465 wouldn't work with Blesta
Indeed it is.
I spend some time yesterday to trace back my steps. I'm using WHM/cPanel by the way. I'm happy to say that I've managed to solve my issue.
Solution
During WHM/cPanel installation. I enabled SMTP restriction as security measure. What this setting does is, it will redirect all connection attempt from PHP to remote SMTP, to internal SMTP. Disabling this fixed it for me.
bigcat's post in Smtp Problem When Using Mandrill & Mailgun was marked as the answer