have multiple email a/cs' but apple mail sending all gmail emails from same account

I have 3 gmail accounts and 2 pop3 a/cs' ( that are piggy backed by the isp to web based/imap email servers) and using Apple Mail.

Emails are correctly received into the correct email address inbox.

However, when I send an email and despite whichever email address I select in the ' From ' box on the Compose page, the emails are always sent from the same email address. This issue only affects the 3 gmail accounts, the other 2 are working ok.

In Mail Prefs I have checked that the box " send new messages from " is marked " account of selected mailbox ".

If I use gmail ' direct ' ( not via apple mail ) emails are sent from the correct account as I obviously have to sign into that particular account.

It's as if I have a default email account from which all emails are sent but the settings seem to be to the contrary.

Appreciative for any help and guidance.