perhaps if you shared the error message someone could stand a chance of interpreting. As it stands you are not going to get anything useful, except requests for more information.

I don't have the full diagnostic text but here is part of it: "connection to outgoing server (SMTP) local host failed. Verify that SMTP settings are correct."

This only happens when something is copied into the send to address or, sometimes, if a link is copied into the text.

When a new email message is generated and the address is typed into the send to or copied from the address book, everything works OK.

Problem resolved. In addition to Gmail (IMAP) I also had Optonline (POP). After deleting Optonline, everything worked.

I think that when the email program was called up, the POP Optonline somehow got in the way.