Can people erase email letters as if they were never sent...?

For example I was emailing someone and had to send a very important letter to them, that letter is erased from my emails - drafts, archives, trash.  Also all the 3 emails before leading to it.  Lets say I send an email and the person and I correspond back and for and the middle of this conversation is deleted - how can that be.  The beginning is there and the end?  Can the person I was emailing somehow erase part of the conversation?  Or can someone else somehow?  This was an important letter and I double checked I sent it, got an email response from Yahoo mail saying it was sent and even made a copy of this letter and sent it to myself.