Searches secret key in the keyring by its e-mail address.
Index of the found key or -1 if nothing was found.
Use this method when you need to find secret key with specified e-mail address.
Note, that some keys can have the same e-mail addresses.
So you need to check that the key found is the one you've searched for, and if no, search again with new value of StartIndex parameter.
Please also note, that this method searches for a given substring in the UserID information, so any substring of a user name can be used as a search criterion (not e-mail address only).