How to Cancel sending an Outlook Read Receipt

I have an email account with btinternet.com that I can download my emails
from within Outlook but (as I don't use their Broadband service) I am barred
from sending to them. If I accidentally try to send a message I can remove it
from my Outbox but I have inadvertently agreed to send a read receipt which
does not appear in my Outbox but does cause Outlook to attempt to connect
every few minutes & the POP server rejects the login.
I understand the receipt will be in the AdministrationQueue but is there a
way I can access that & delete it? Also, preferably stop Outlook from ever
trying to send to that POP server in future (but still receive from it).
Thanks.