The error can be simulated on various different systems.

Because I can't reproduce this issue (J3.9.5, PHP7.3) with PHP-Mail or SMTP (SSL/TLS; Authentication) settings and feature Send Test Mail on 3 test sites: Could you please provide more system informations and testing instructions?

The problem can be limited to one PHP problem, because several installations are affected. As soon as PHP 7.3 is used, the mail dispatch does not work anymore. If you switch back to 7.1 or 7.2, sending the mail works without problems. It occurs with all variants of the mail dispatch! (Sendmail, SMTP)

It has been tested with Joomla 3.9.4 and 3.9.5

Please search Joomla Forums. Here are some threads regarding this issue:

https://forum.joomla.org/viewtopic.php?f=706&t=970270
https://forum.joomla.org/viewtopic.php?f=706&t=968659

I am familiar with the two issues, but they do not solve the problem. Since the error occurs with different systems it is to be assumed that this is an error within PHP. In the parallel discussion in the Glip Channel D_A_CH to this problem different approaches were already mentioned.

Global sending of emails disabled - no
Switching the sending methods - sendmail to SMTP etc. without success.
Several providers were tested, at 1&1 (1x OK 1x failed), Goneo (1x OK 1x failed), Mittwald (30 x failed).

The support of Mittwald said that the PHPMailer 5.2 does not work with PHP 7.3. The same message came from another community portal which does not work with Joomla.

Closed as having Pull Request.