If you have habits with regard to note keeping, yu can find the information on our computer or on your backups.
Example:
One user has the habit always make a TXT file with his user name and password for every e-mail and other account ever made at any time. He always made a note in a TXT file. Sad to say, he does not remember how he named the text files he created. He only recalls that he always used the TXT type and never the DOC type for the files
Solution? He turned on the indexing service for his PC and started to look for text files with the @ character. He found a lot. He had to read each one. some
Of course, if one tends to have bad habits, one can pay other people a lot of money to arrange the disorder.
Side Bar:
In the USA that are a number of wrongful convictions due to improper use of forensic evidence. This could possibly happen with data gathered from e-mail.