@SHust noticed that Civi CID51532370 yields an error message: Email is not valid when trying update the email address. The only difference I can see between the existing address and what the donor has asked us to replace it with is that the former has _o_ and the latter has -o-
If the validation rules for email addresses can be modified to accommodate this, that would be great.
Zendesk #965271 is the ticket related to this.