If the server is not responding, then you have the wrong host name, or wrong port, or there’s a firewall in the way—stay off of TCP port 25 if your mail provider even still supports client access on that port, and for just this reason—or less likely there’s a problem with the server, or less likely there’s a problem with your DNS server settings.
Your mail provider will have the specific host and port requirements. These vary by provider. You’re looking for the send/SMTP/ESMTP settings, port, and connection-security details, here.
Usual cause here is a bad SMTP server setting, and you’ve probably been making changes in the receive/read/POP/IMAP settings and not the send/SMTP/ESMTP server settings. Mail has two groups of settings including the server snd port and security settings and credentials, and both groups have to be correct.
If you can’t send or receive emails on your iPhone, iPad, or iPod touch - Apple Support