mail constantly "sending" mails
On my Macbook, in mail, at the bottom of the mailboxes column the message "sending x mails out of y messages" constantly appears. (it seems to be that x is normally y-3. e.g. at the moment 42 out of 45).
I am on 10.11.6 El Capitan.
Is this significant to anything please? Battery life? Security?
Or do I "upgrade' to Sierra and hope!?
Any help gratefully received.
Thanks
Bill
OS X El Capitan (10.11.6)