Sending test email fails with SMTP-Setup

Replies: 0

Hi everybody,
I am facing a weird issue with the WP Mail SMTP settings. I try to use a relayed SMTP server with authentication. Using exactly the same connection and authentication details in WP Mail which is working on another platform (not WP) lets the “send test mail” fail after a timeout with the following error:

Error while sending QUERY packet. PID=26318 in /www/htdocs/ftp-user/domain.tld/wp-includes/wp-db.php on line 2030

Warning: Cannot modify header information - headers already sent by (output started at /www/htdocs/ftp-user/domain.tld/wp-includes/wp-db.php:2030) in /www/htdocs/ftp-user/domain.tld/wp-admin/includes/misc.php on line 1252

Warning: Cannot modify header information - headers already sent by (output started at /www/htdocs/ftp-user/domain.tld/wp-includes/wp-db.php:2030) in /www/htdocs/ftp-user/domain.tld/wp-admin/admin-header.php on line 9

Warning: Cannot modify header information - headers already sent by (output started at /www/htdocs/ftp-user/domain.tld/wp-includes/wp-db.php:2030) in /www/htdocs/ftp-user/domain.tld/wp-includes/option.php on line 961

Warning: Cannot modify header information - headers already sent by (output started at /www/htdocs/ftp-user/domain.tld/wp-includes/wp-db.php:2030) in /www/htdocs/ftp-user/domain.tld/wp-includes/option.php on line 962

When using WP default (without setting up SMTP) sending test mail works fine, if the receipient does not have the same domain as the email it is sent from. But if the domain of the sender and the receipient is the same, system displays that test email has been send successfully, but neither the inbox nor the mail log shows any activity (as email has never being sent).

Unfortunately no smtp log in WP is available.

I have spent hours on that to solve the issue with no success. Can anybody help with this?