We have been trying to read Bob's emails for 5 years. 
We sniffed all his traffic and reversed his POP3 encrypted protocol.
He is using a good block cipher, CBC mode and unique initialization vectors for each command.
We easily found Alice password and decrypted her mails but we never found the server key or his passwords.
Until one day when Bob had very bad luck and we were able to decrypt all his mails. 
Get bobpop.cap, discover what happened and read his mails.