504 Gateway timeout with Office365 SMTP

Replies: 0

Hello,

I’m having trouble configuring the Office 365 SMTP on a server. Despite using the correct settings (it works just fine on another machine) every time I try to send a test message I only get a “504 Gateway timeout” error from nginx. Switching between SSL and TLS doesn’t help either.
The working settings on the other machine are smtp-mail.outlook.com with TLS on port 587.
I’ve read several times on other issues here that you requested SMTP debug output but I can’t figure out how to enable it. WordPress debugging is enabled:

define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);

But nothing is showing up in the Apache / PHP error logs. How do I see what’s going wrong here? Interestingly I receive the error message from nginx despite the website being served by Apache.

Technical details:
Web server: Apache
PHP: 7.2.15
Wordpress: 5.1.1
WP Mail SMTP: 1.4.2

Thanks,
Stefan