authentication failure [SMTP: STARTTLS failed (code: 220, response: 2.0.0 Start TLS)]
This question appears to be the same as Outbound mail fails after upgrading to 4.7.1 tl/dr i suspect your probably running on php5.6 or similar version and the ssl certificate for smtp.mailgun.org is not valid or is a CACert or similar and PHP5.6 is saying i can't validate certificate.