Changing account login email stops mail Send, goes to Outbox. Receive, Contacts, Cal ok.

After changing the login email address of my account to a different Alias, em Client will not send emails.  They just sit in the Outbox.  Trying to resend also fails.

The web mail app operates perfectly logging in with the new email alias.

I have tried:
 - Deleting and reinstalling the account in em Client.
 - Setting the original login up as an Alias in em Client, then selecting it when sending an email – It Worked!!

I Deleted and reinstalled the account as an Exchange account, with the new login credential, now the message appears to send and goes to the Sent folder, but the email is never delivered to recipient.

Any advice appreciated.