You need to access your email settings within the app. You need to know your email providers requirements. smtp needs to know the address, for instance smtp.google.com

You need to know the port number, for instance 25, 465

You need to know whether encryption is required, and if it is, is it tsl or ssl and set it correctly.

Check those settings, google your email providers (your domain name) requirements, and change them as required.

If you've done all this and still have problems, remove and reinstall your email client, sometimes things just get corrupted. Or try another email client, Thunderbird is the old fall back favorite.

This video will sort of familiarize you with the mail app and settings.