Whilst either one of my VPN dedicated IP addresses is running I cannot send. With same VPN provider’s other servers or no VPN I can send fine.
VPNs are renowned for problems with using most email clients including eM Client unless you are very lucky. This is due to the nature of the IP addresses changing & VPN servers etc. I normally don’t recommend using them to avoid issues, but if you want to use one see this EMC thread where one user @skybat uses Hideaway from Firetrust which apparently works fine with eM Client.
(Will not work when VPN is used)