We should target any fixes for this to master-2.19 branch and merge into master before releasing 2.20.0.

Here is the snippet from the error log:

[Tue Mar 05 04:19:24.628039 2019] [:error] [pid 28020] [client 172.31.47.148:37510] 
PHP Fatal error:  Uncaught PHPMailer\\PHPMailer\\Exception: SMTP Error: Could not connect to SMTP host. in /.../www/vendor/phpmailer/phpmailer/src/PHPMailer.php:1947
Stack trace:
#0 /.../www/vendor/phpmailer/phpmailer/src/PHPMailer.php(1774): PHPMailer\\PHPMailer\\PHPMailer->smtpConnect(Array)
#1 /.../www/vendor/phpmailer/phpmailer/src/PHPMailer.php(1516): PHPMailer\\PHPMailer\\PHPMailer->smtpSend('Date: Tue, 5 Ma...', 'The following i...')
#2 /.../www/vendor/phpmailer/phpmailer/src/PHPMailer.php(1352): PHPMailer\\PHPMailer\\PHPMailer->postSend()
#3 /.../www/core/email_api.php(1395): PHPMailer\\PHPMailer\\PHPMailer->send()
#4 /.../www/core/email_api.php(1220): email_send(Object(EmailData))
#5 /data/srv/instances/dba740bb-1a25-40a2 in /.../www/vendor/phpmailer/phpmailer/src/PHPMailer.php on line 1947, referer: https://mantis/bug_report_page.php

I haven't proven that TLS is the cause of the 3rd issue.

Here are the PHPMailer 6 upgrade guide:
https://github.com/PHPMailer/PHPMailer/blob/master/UPGRADING.md

are the latest PHPMailer releases working for you?

I have not experienced any issues on my end since I started testing PHPMailer 6 about one year ago. This code has been running on our tracker since November 2018, and I have not noticed any problems there either (I'm receiving notifications every day).

PHPMailer::ValidateAddress() seems to be replaced by PHPMailer::validateAddress().

I know, but this is not an issue as PHP Function names are case-insensitive so I didn't think it was worth the effort to adapt the code to the new camelCase naming. I can do it if you want.

catch ( phpmailerException $e ) { seems to be replaced by catch ( Exception $e ) { # PHPMailer Exception - two occurences

I missed those during migration, will fix.

StartTLS fails when config is set to use TLS.

I don't have a config to test this. Can you provide more information / steps to reproduce ?

Sorry I pushed to the wrong remote and source integration picked the commits up and marked the issue as fixed.

See PR https://github.com/mantisbt/mantisbt/pull/1475 (does not include fix for TLS issue)

1 & 2 are fixed.

Please open a separate issue for 3 with additional details if your TLS issue comes up again.

@dregad It turns out the issue is not related to TLS. But I opened bug 0025582 and opened a corresponding bug in PHPMailer with the fix I found. However, I expect the PHPMailer team to find a better fix.