[Exim] To verify or not...

...that is the question.

I have just installed Exim 4 and am using sender verify. Examining
the logs, I can see a couple of messages that were refused due to
being unable to verify the sender. Admitedly these messages look like
spam, and anything I can do to reduce spam the better.

But, is there any situation that anyone knows of when a legitimate
address might get refused due to Exim not being able to verify the
sender address? The last thing I want to do is block legit e-mail.

I am using mailscanner and spamassassin so this also helping me to
identify (rather than reduce) spam so would I be better accepting all
mail and letting spam assassin do it's work and basically let the
user decide what to do with it?

Please help

Chris Mills