dI am a user in China. I don't know why even vpn can receive messages but can't send them. Even VPN can't receive messages but send them.

1

I am a user in China. I don’t know why even vpn can receive messages but can’t send them. Even VPN can’t receive messages but send them.

My account cannot connect to the server, it shows that it is connecting [works on desktop and Android&VPN]
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Which server are you using? Some VPN providers block email-related ports, unfortunately.
If you’re using a Chatmail then this should not affect you because those run on non-default email ports. See Could provide a set of non-standard ports on nine.testrun.org - #3 by adbenitez.