The message told staff: “I would like to take this opportunity to thank you and wish you all the best for the future.”
It also included instructions regarding procedures for leaving the company, such as handing back company equipment and confidentiality.
The email had been intended for one worker who was leaving that day.
An Aviva Investors spokesman insisted: “People were pretty quickly aware of the fact that this was a mistake … I don’t believe any of our staff would have seen it really as anything other than the mistake that it was.”
Aviva announced in January that it would be shedding 160 jobs.
There has been no comment on why the intended recipient of the email was leaving Aviva.