Cheap Web Space, Domain Name:
Web Space With E-Mail, PHP/MySQL.

Symptom

You have enabled the Nextcloud Mail App and can use other email accounts without any problem. Only to Gmail, no connection can be established.

Cause

Gmail prefers to communicate over IPv6. Our hosts publish IPv6 routes but are currently not responding to specific requests because security software cannot (yet) block certain attack vectors. To keep the system safe, network traffic is completely blocked at this point.

Solution

Add this directive to the Nextcloud config.php file:

'app.mail.transport' => 'php-mail'

Thus, the app no longer uses the direct SMTP login, but the PHP mail functions. In turn, PHP is configured to allow connection to all other mail servers, including Gmail.

Overview of frequently asked questions