Hello, as described Teltonika devices seem to have difficulties with specific TLS/SSL constellations. There is another post in the forum: https://community.teltonika-networks.com/45780/unable-to-send-email-secure-ssl-tls
My provider is domainfactory and I have exactly the same issue. Seriously I don’t think that all the mail providers have bugs. Also the port 465 works on any other device without problems. The RUT is the only device which fails.