You may have been directed by us or your hosting provider to update your incoming and outgoing email server settings. This guide walks you through how to do this in eM Client on Windows desktop for an existing account that has already been added to eM Client.
Guide
Step 1. In the list on the left, right click on the account and select "Account Settings..." from the dropdown:

Step 2: A pop-up will appear. Click on the "IMAP" tab and change the "Host" field:

Step 3: Now click on the "SMTP" tab and change the "Host" field:

Step 4: Click the "Save & Close" button in the top left of the window.
If you have any issues, open a new ticket here.