PHPMailer SMTP Error: Could not connect to SMTP host - email

Question

Having a strange problem with PHPMailer where it's showing an SMTP connection error, as seen below. What could cause this? It works when I use the Gmail SMTP server, and also works with the local mail function. Roundcube on the server sends fine. It just seems to be PHPMailer. All settings are correct, to the best of my knowledge. It's using TLS and I've tried different ports. The correct ports show the following error:
2016-05-05 21:58:47 Connection: opening to mail.myserver.com:587, timeout=300, options=array (
)
2016-05-05 21:58:47 Connection: opened
2016-05-05 21:58:52 SMTP -> get_lines(): $data is ""
2016-05-05 21:58:52 SMTP -> get_lines(): $str is "220 sys.myserver.com ESMTP Exim 4.72 Thu, 05 May 2016 17:58:50 -0400
"
2016-05-05 21:58:52 SERVER -> CLIENT: 220 sys.myserver.com ESMTP Exim 4.72 Thu, 05 May 2016 17:58:50 -0400
2016-05-05 21:58:52 CLIENT -> SERVER: EHLO localhost
2016-05-05 21:58:52 SMTP -> get_lines(): $data is ""
2016-05-05 21:58:52 SMTP -> get_lines(): $str is "250-sys.myserver.com Hello ***********]
"
2016-05-05 21:58:52 SMTP -> get_lines(): $data is "250-sys.myserver.com Hello cpc68331-cdif16-2-0-cust906.5-1.cable.virginm.net [86.3.207.139]
"
2016-05-05 21:58:52 SMTP -> get_lines(): $str is "250-SIZE 52428800
"
2016-05-05 21:58:52 SMTP -> get_lines(): $data is "250-sys.myserver.com Hello cpc68331-cdif16-2-0-cust906.5-1.cable.virginm.net [86.3.207.139]
250-SIZE 52428800
"
2016-05-05 21:58:52 SMTP -> get_lines(): $str is "250-PIPELINING
"
2016-05-05 21:58:52 SMTP -> get_lines(): $data is "250-sys.myserver.com Hello ***************************]
250-SIZE 52428800
250-PIPELINING
"
2016-05-05 21:58:52 SMTP -> get_lines(): $str is "250-AUTH PLAIN LOGIN
"
2016-05-05 21:58:52 SMTP -> get_lines(): $data is "250-sys.myserver.com Hello ****************************]
250-SIZE 52428800
250-PIPELINING
250-AUTH PLAIN LOGIN
"
2016-05-05 21:58:52 SMTP -> get_lines(): $str is "250-STARTTLS
"
2016-05-05 21:58:52 SMTP -> get_lines(): $data is "250-sys.myserver.com Hello ******************]
250-SIZE 52428800
250-PIPELINING
250-AUTH PLAIN LOGIN
250-STARTTLS
"
2016-05-05 21:58:52 SMTP -> get_lines(): $str is "250 HELP
"
2016-05-05 21:58:52 SERVER -> CLIENT: 250-sys.myserver.com Hello ***************]
250-SIZE 52428800
250-PIPELINING
250-AUTH PLAIN LOGIN
250-STARTTLS
250 HELP
2016-05-05 21:58:52 CLIENT -> SERVER: STARTTLS
2016-05-05 21:58:52 SMTP -> get_lines(): $data is ""
2016-05-05 21:58:52 SMTP -> get_lines(): $str is "220 TLS go ahead
"
2016-05-05 21:58:52 SERVER -> CLIENT: 220 TLS go ahead
2016-05-05 21:58:52 SMTP Error: Could not connect to SMTP host.
2016-05-05 21:58:52 CLIENT -> SERVER: QUIT
2016-05-05 21:58:53 SMTP -> get_lines(): $data is ""
2016-05-05 21:58:53 SMTP -> get_lines(): $str is "����221 sys.myserver.com closing connection
"
2016-05-05 21:58:53 SMTP -> get_lines(): $data is "����221 sys.myserver.com closing connection
"
2016-05-05 21:58:53 SMTP -> get_lines(): $str is ""
2016-05-05 21:58:53 SERVER -> CLIENT: ����221 sys.myserver.com closing connection
2016-05-05 21:58:53 SMTP ERROR: QUIT command failed: ����221 sys.myserver.com closing connection
----------
2016-05-05 21:58:53 Connection: closed
2016-05-05 21:58:53 SMTP connect() failed. https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting
{"success":1,"id":"3391"}

Answer 1

This is covered in the troubleshooting guide. It's failing immediately after STARTTLS, which usually indicates that your mail server is publishing an invalid SSL certificate, either expired, self-signed or doesn't match the host name. The real solution is to fix the cert, but the guide tells you how you can disable verification.
Presumably gmail is working because it doesn't have a broken SSL config, and roundcube is probably submitting to localhost without TLS (via mail()), so it won't encounter the problem.