Not looking for a server as I use fastmail currently but a client would be nice. I am currently using spark on iOS and Mac OS and just fastmail website on my Linux desktop. I’m realizing spark is nothing special and would rather not renew my subscription.
Thing like send later. You can do it in a mail client but it requires the client to be running. It you implement it on the server you can guarantee that the email gets send on a specific day/time.
Spark offers collaboration on messages. So for example your team can add comments on an email.
Etc.
Outlook server providers have this feature, if it’s really crucial.
…You can reply to an email. It’s not a special feature, it’s how email works.
If you mean to annotate something live together with other people, there are office tools for that. This is unrelated to email.