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.
What features?
Manage your mail on multiple clients, as in native software that runs on a device. If your client always deletes upon fetching your mail, another device won’t see it.
You can leave the messages on the server and use IMAP to look at them from multiple devices in non-destructive ways.
If you must eventually move the mail out of the server then you can use IMAP devices in combination with one POP device that serves as the main mail archive, but only deletes the email from the server on a delay – say, 30 days. During those 30 days the IMAP clients continue to see the email and the POP client won’t re-download the ones it already has. After 30 days a message can only be found on the device that uses POP.
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.