I hope this configuration helps someone. Should be added to documentation. Worked for us:
"mail": {
"from": "<EMAIL>",
"transport": "SMTP",
"options": {
"service": "Mailgun",
"host": "smtp.mailgun.org",
"port": 465,
"secure": true,
"auth": {
"user": "<USER>",
"pass": "<PASS>"
}
}
},
"tls": {
"rejectUnauthorized": false
},