Here is Diagnostic report, enjoy…
Mailer: postsmtp
HostName: domain.com
cURL Version: 7.74.0
OpenSSL Version: OpenSSL/1.1.1n
OS: Linux host.domain.com 5.10.0-21-amd64 #1 SMP Debian 5.10.162-1 (2023-01-21) x86_64
PHP: Linux 8.0.27 C
PHP Dependencies: iconv=Yes, spl_autoload=Yes, openssl=Yes, sockets=Yes, allow_url_fopen=Yes, mcrypt=No, zlib_encode=Yes
WordPress: 6.1.1 en_AU UTF-8
WordPress Theme: GeneratePress
WordPress Plugins: Query Monitor, Post SMTP, Advanced Database Cleaner, Async JavaScript, Classic Editor, Contact Form 7, Delete Expired Transients, Lazy Images Without Jetpack, Limit Login Attempts Reloaded, MainWP Dashboard, Minify HTML, Plugin Logic, Scripts-To-Footer, Slash Admin, Speed Booster Pack, WP Crontrol, WP-ServerInfo, WP Super Cache
WordPress wp_mail Owner: /home/****/domain.com/wp-content/plugins/post-smtp/Postman/PostmanWpMailBinder.php
WordPress wp_mail Filter(s): PostsmtpMailer->get_mail_args
WordPress phpmailer_init Action(s): wpcf7_phpmailer_init, PostsmtpMailer->phpmailer_smtp_init
Postman: 2.4.1
Postman Sender Domain (Envelope|Message): domain.com | domain.com
Postman Prevent Message Sender Override (Email|Name): Yes | Yes
Postman Active Transport: SMTP (smtp:none:none://localhost:25)
Postman Active Transport Status (Ready|Connected): Yes | Yes
Postman Deliveries (Success|Fail): 6 | 0
Postman Email Log (Enabled|Limit|Transcript Size): No | 1 | 1
Postman Stealth Mode: Yes
Postman File Locking (Enabled|Temp Dir): No | /tmp