STARTTLS not working

When using an external SMTP server to send emails it fails to send any emails due to problems with STARTTLS. I have "fixed" my version by placing:

return false;

immediately after the function starttls() open braces in includes/functions_messages.php, thus effectively disabling STARTTLS. Without that the code attempts to start a TLS session but this appears in the MTA logs:

postfix/smtpd[19550]: lost connection after STARTTLS from hostname.example.com[x.x.x.x]

This is the phpBB backtrace:

Email error

» EMAIL/SMTP

/forum/adm/index.php

Could not get mail server response codes.

Backtrace

Connecting to mta.example.com:25

LINE: 1022 <- 220 mta.example.com ESMTP Postfix 

# EHLO webserver.example.com

LINE: 1371 <- 250-mta.example.com 

LINE: 1371 <- 250-PIPELINING 

LINE: 1371 <- 250-SIZE 52428800 

LINE: 1371 <- 250-VRFY 

LINE: 1371 <- 250-ETRN 

LINE: 1371 <- 250-STARTTLS 

LINE: 1371 <- 250-AUTH PLAIN LOGIN 

LINE: 1371 <- 250-AUTH=PLAIN LOGIN 

LINE: 1371 <- 250-ENHANCEDSTATUSCODES 

LINE: 1371 <- 250-8BITMIME 

LINE: 1371 <- 250 DSN 

# STARTTLS

LINE: 1416 <- 220 2.0.0 Ready to start TLS 

# AUTH PLAIN

The MTA is a Centos 7 server running Postfix version 2.10.1
The web server is also Centos 7 running Apache version 2.4.6 with PHP version 5.6.20

Detailed logs are available if required.

Regards,
Chris