Create a new Roundup instance.
In there, create a user whose username is the
same as one of the users of the local machine.
Don't assign an e-mail address to that user.
Now, have that user send a message to the
Roundup mail gateway.

This message will have a "From" field something
like
    From: Selected Person Quite at Random <spqr1>

In roundupdb.Database.uidFromAddress, we
first of all look for a user whose "address"
property equals "spqr1". There will be none.
(Recall that we assigned no e-mail address
to the Roundup user called spqr1.) So then
we try to create a new user called spqr1.
This fails because there already is one.
Therefore, the mail gateway will bounce the
message.

This seems sub-optimal. I think one of two
things should happen. Either the existing
user should be used, or a new user should
be created with a name like "spqr1@localhost".
I prefer the first option.

This is clearly not a serious problem.