@damingzi
https://ww.wp.xz.cn/plugins/postman-smtp/other_notes/
Could not open socket
Your host may have installed a firewall between you and the server. Ask them to open the ports.
Your may have tried to (incorrectly) use SSL over port 587. Check your encryption and port settings.
For anyone else searching and ending up here…
We’ve been having this problem — turned out to be a classic issue on many shared hosting platforms, where a single security certificate covers the whole server. Email clients and web browsers will therefore throw up a warning and you can save an exception and forget about it. This plugin doesn’t throw up a warning, it just rejects the certificate and blocks it. The solution (in our case) was to specify the generic server domain name instead of the website’s (e.g. mail.mysharedhostingserver.com instead of mail.myowndomain.com).
BTW this is usually not an issue if the email and website are hosted on the same server, but as soon as you’re going outside the same cPanel (or similar) all the security checks kick in.
