Getting "... emails were skipped during your last import"

I'm creating my second email list via importing a CSV file. After import I'm told that 3 emails were skipped. On exporting the skipped list I find:

a) Only two, not three emails, are in the skipped list.
b) Of the two emails listed, one is listed in my other email list as a bounce, so that's OK. The second email, however, is NOT listed anywhere as "Unsubscribed / Bounced /Marked as spam" (the reason given for the rejection). That email has never been anywhere near sendy.

Any suggestions as to why a properly formatted new email address would be rejected as "Unsubscribed / Bounced /Marked as spam"?