Cannot send gmail emails

The other day my wife was sending emails via eM Client with no problem.  All of a sudden sent emails started to go onto an Outbox folder and not to the intended recipient.  Emails can, however, still be sent from the Gmail website. This is extremely frustrating to say the least. The error message that displays is " {SMTP} An attempt to connect to failed.  This could be caused by temporary server unavailability or incorrect settings. Do you want to check the settings?"  The settings are automatically set up and in any case they were working ok previously.
Can anyone suggest the most likely cause of the malfunction and how to make the necessary corrections please?

Hello Gordon,

In this case if it’s an IMAP account I’d recommend removing the account and re-configuring it in Menu > Tools > Accounts. If the server changed any settings when the problem started, this usually fixes it.