I can receive but not send e-mails in eM Client when I’m on foreign WI-FI. My home wi-fi works fine, but if I’m on a guest Internet connection e.g. at a university where I teach, I can only receive. Webmail works fine.
The most likely reason is that your email provider does not accept accessing their SMTP (sending) server from foreign networks. At home you are connected to their network, so it works fine. You could contact them and ask about that. If it is just one Wi-Fi that you can’t use, then it is possible that that one has blocked SMTP protocols.
The other option you could do Thomas is if you have access to your email via a browser using webmail.
I had a friend who had that issue when he went overseas and wasn’t allowed to send email via his company smtp server via EMClient out of the country. So he used to access his email via webmail in a browser and bcc himself on any sent email. Then when he got back home, he put that email in his sent box. Its not ideal but worked for him. Just a thought if you have Webmail acess to your email