Even though I don’t fell you would need a diagnostic test because this error is happening on ALL sites and (i tried) with at least two different contact form plugins. But if it helps you and us, here you go
Mailer: postsmtp
HostName: bbbeta.instawp.xyz
cURL Version: 7.68.0
OpenSSL Version: OpenSSL/1.1.1f
OS: Linux productionfra100.instawp.xyz 5.4.0-153-generic #170-Ubuntu SMP Fri Jun 16 13:43:31 UTC 2023 x86_64
PHP: Linux 8.1.20 C.UTF-8
PHP Dependencies: iconv=Yes, spl_autoload=Yes, openssl=Yes, sockets=Yes, allow_url_fopen=Yes, mcrypt=No, zlib_encode=Yes
WordPress: 6.3.1 en_US UTF-8
WordPress Theme: GeneratePress
WordPress Plugins: Beaver Builder Plugin (Developer Version), Calculated Fields Form, Fluent Forms, Forminator, GP Premium, Popup Maker, Post SMTP
WordPress wp_mail Owner: /home/nufitupawa4487/web/bbbeta.instawp.xyz/public_html/wp-content/plugins/post-smtp/Postman/PostmanWpMailBinder.php
WordPress wp_mail Filter(s): wp_staticize_emoji_for_email, PostsmtpMailer->get_mail_args
WordPress phpmailer_init Action(s): PostsmtpMailer->phpmailer_smtp_init
Postman: 2.6.1
Postman Sender Domain (Envelope|Message): kjkjkhen.de | netzmaedchen.de
Postman Prevent Message Sender Override (Email|Name): No | No
Postman Active Transport: SMTP (smtp:tls:plain://w0ccgg.kasserver.com:587)
Postman Active Transport Status (Ready|Connected): Yes | Yes
Postman Deliveries (Success|Fail): 6 | 0
Hope you can help, as others are having the same problem.
Regards