SMTP Error: Could not connect to SMTP host

Hello,

OS: CentOS 7.3.1611

Apache: 2.4.6

PHP: 7.0.23

Recently migrated my WHMCS installation from a CentOS6/PHP5.4 server to the above spec'd server.

Quadruple-checked SMTP settings in General > Mail.

Firewalls on both WHMCS and mail server disabled.

Can confirm DNS/Net connectivity between servers.

POP import is working correctly, SMTP is not.

Enabled smtp_debug in configuration.php

URL's have been obfuscated:

mail.whmcs-domain.com = Mail address for WHMCS domain (essentially, mail.server.com)

mail.server.com = WHM/cPanel server

http://www.whmcs-domain.com = WHMCS installation domain

2017-09-13 20:09:57	Connection: opening to mail.whmcs-domain.com:587, timeout=300, options=array ( ) 
2017-09-13 20:09:57	Connection: opened 
2017-09-13 20:09:57	SMTP -> get_lines(): $data is "" 
2017-09-13 20:09:57	SMTP -> get_lines(): $str is "220-mail.server.com ESMTP Exim 4.89 #1 Wed, 13 Sep 2017 13:09:57 -0700 " 
2017-09-13 20:09:57	SMTP -> get_lines(): $data is "220-mail.server.com ESMTP Exim 4.89 #1 Wed, 13 Sep 2017 13:09:57 -0700 " 
2017-09-13 20:09:57	SMTP -> get_lines(): $str is "220-We do not authorize the use of this system to transport unsolicited, " 
2017-09-13 20:09:57	SMTP -> get_lines(): $data is "220-mail.server.com ESMTP Exim 4.89 #1 Wed, 13 Sep 2017 13:09:57 -0700 220-We do not authorize the use of this system to transport unsolicited, " 
2017-09-13 20:09:57	SMTP -> get_lines(): $str is "220 and/or bulk e-mail. " 
2017-09-13 20:09:57	SERVER -> CLIENT: 220-mail.server.com ESMTP Exim 4.89 #1 Wed, 13 Sep 2017 13:09:57 -0700 220-We do not authorize the use of this system to transport unsolicited, 220 and/or bulk e-mail. 
2017-09-13 20:09:57	CLIENT -> SERVER: EHLO www.whmcs-domain.com 
2017-09-13 20:09:57	SMTP -> get_lines(): $data is "" 
2017-09-13 20:09:57	SMTP -> get_lines(): $str is "250-mail.server.com Hello www.whmcs-domain.com [104.250.128.107] " 
2017-09-13 20:09:57	SMTP -> get_lines(): $data is "250-mail.server.com Hello www.whmcs-domain.com [104.250.128.107] " 
2017-09-13 20:09:57	SMTP -> get_lines(): $str is "250-SIZE 52428800 " 
2017-09-13 20:09:57	SMTP -> get_lines(): $data is "250-mail.server.com Hello www.whmcs-domain.com [104.250.128.107] 250-SIZE 52428800 " 
2017-09-13 20:09:57	SMTP -> get_lines(): $str is "250-8BITMIME " 
2017-09-13 20:09:57 SMTP -> get_lines(): $data is "250-mail.server.com Hello www.whmcs-domain.com [104.250.128.107] 250-SIZE 52428800 250-8BITMIME " 
2017-09-13 20:09:57	SMTP -> get_lines(): $str is "250-PIPELINING " 
2017-09-13 20:09:57	SMTP -> get_lines(): $data is "250-mail.server.com Hello www.whmcs-domain.com [104.250.128.107] 250-SIZE 52428800 250-8BITMIME 250-PIPELINING " 
2017-09-13 20:09:57	SMTP -> get_lines(): $str is "250-STARTTLS " 
2017-09-13 20:09:57	SMTP -> get_lines(): $data is "250-mail.server.com Hello www.whmcs-domain.com [104.250.128.107] 250-SIZE 52428800 250-8BITMIME 250-PIPELINING 250-STARTTLS " 
2017-09-13 20:09:57	SMTP -> get_lines(): $str is "250 HELP " 
2017-09-13 20:09:57	SERVER -> CLIENT: 250-mail.server.com Hello www.whmcs-domain.com [104.250.128.107] 250-SIZE 52428800 250-8BITMIME 250-PIPELINING 250-STARTTLS 250 HELP 
2017-09-13 20:09:57	CLIENT -> SERVER: STARTTLS 
2017-09-13 20:09:57	SMTP -> get_lines(): $data is "" 
2017-09-13 20:09:57	SMTP -> get_lines(): $str is "220 TLS go ahead " 
2017-09-13 20:09:57	SERVER -> CLIENT: 220 TLS go ahead 
2017-09-13 20:09:57	SMTP Error: Could not connect to SMTP host. 
2017-09-13 20:09:57	CLIENT -> SERVER: QUIT 
2017-09-13 20:09:58 SMTP -> get_lines(): $data is "" 
2017-09-13 20:09:58	SMTP -> get_lines(): $str is "221 mail.server.com closing connection " 
2017-09-13 20:09:58	SERVER -> CLIENT: 221 mail.server.com closing connection 
2017-09-13 20:09:58	Connection: closed 
2017-09-13 20:09:58	SMTP Error: Could not connect to SMTP host.
Email Sending Failed - SMTP Error: Could not connect to SMTP host.

I've seen the rest of the threads here on the forums and out in the wild regarding this error, and for the most part they relate to mismatched SSL certificates on the mail host. This doesn't seem to be the case, but I've also attempted to modify class.phpmailer.php as recommended, yet the issue persists.

Been banging my head against this for a bit here, any help would be much appreciated.

Thank you.