Bill wrote:

What can cause an e-mail placed in the Outbox with a normal Send
operation to remain there causing multiple sends of the same e-mail?
It is necessary to open the Outbox and delete its contents in order
for the problem to clear.

A couple of possibilities a

- You are trying to send an e-mail whose size exceeds the maximum
allowed by your e-mail provider. In the Size column (add it if
missing), check what is the actual size of the e-mail that you intend
to send. Then check what is the max size per e-mail in the quotas
established by your e-mail provider.

- You are interrogating your outbound e-mail traffic by an anti-virus or
anti-spam program. Disabling its interrogation and retest.