Email Aliases - Error message when sending - NEW

Email Aliases:When I try to send out an email from one of my alias accounts, I get an Error Message saying that the “Mailbox is Unavailable”. This is new - I have not changed anything on my accounts and have ensured that the mailboxes from the original gmail accounts is still accessible. Is there something else I should be checking? It’s never happened before.

SEE: https://forum.emclient.com/emclient/topics/cant-send-email-from-an-alias-email-address