Answering my own question:

Vista proved not to be the significant variable in the problem.  The
troubleshooting documentation led me to this query:

select * from sys.transmission_queue

... which yielded this result:

An exception occurred while enqueueing a message in the target queue. Error:
15517, State: 1. Cannot execute as the database principal because the
principal "dbo" does not exist, this type of principal cannot be
impersonated, or you do not have permission.

... which is an error fixed by executing:

EXEC sp_changedbowner 'sa'