I have contacted the server but have not heard anything back, however this also happens with my iPhone application
If it is happening on all devices, then you will need to get back to the provider as their server may be offline, or they changed some settings.
If it is just happening in eM Client, as a first step test, can you completely disable any anti-virus, firewall, proxy or VPN, then try again. You can also restart your router and see if that helps.
If that doesn’t help, and you can send from other devices, in eM Client click on the Open account settings in the error, and send a screenshot of the SMTP tab. Maybe there is an incorrect setting.
Same Problem facing from last few days
Yes I too suspect it might be an SMTP setting wrong somewhere as @Gary suggested above (and need a SMTP tab screenshot), as i can definitely telnet and ping “smtp.copper.net” & “mail.copper.net” (no problems) so i don’t think there is a smtp mail server issue at the isp and more of a mail client setup or some other computer software issue locally.