A send to B an encrypted message:
- the mail is encryted with the public key of B
- if A has the public key of B in his personal adress book or the domino names directory, where B and his public key is registered and available
- B decrypts the reveived mail with his private key
- A stores the send mail with his own public/private key (im not sure which key excactly)
So, if you ask, why you should send your public key: if the other guy has no access to the domino directory, where you are registered, than he has no access to your public key. Therefore you must send him an email with your public key. This information will be saved in the personal addressbook of the other guy, so he can send you encrytped mail in the future.
look at google.com for the public&private key procedure to find more.