— if emails are saved in Outlook

You may need to refer to such a mail while writing in jira or wiki or MSOffice.

• identifier — use Subject of the mail to specify it. This way, you or another person can search for the subject in Outlook, across all folders.

— if emails are saved in gmail

You may need to refer to such a mail in blog or recoll

• identifier — I think timestamp is effective because All-mail folder is sorted by timestamp