We are currently performing an email migration. Unfortunately one of the side effects in the migration is the sending of old meeting invites. You may safely ignore or delete the invites. The migration is almost complete and this will be a one-time issue.
We apologize for the annoyance and concerns that some of you have expressed. This is not a virus and will end after the accounts are migrated and fully synced. Thank you for your patience.
UPDATE: Confirmed that this was resolved the morning of the 30th.