I am trying to write a service that provide user a subdomain as mailbox.
So I need to implement IMAP server on my own.
I notice mimekit/mailkit may help me. (At least they can help me test my server.)
It is possible to implement a minimum IMAP Protocol (server side) using mimekit/mailkit?
I would say that MimeKit would certainly be helpful in that a significant part of writing an IMAP server (or even a POP3 or SMTP server) is that you will at some point need to deal with parsing messages and/or headers and MimeKit is a perfect solution for that.
There are parts of MailKit which might be helpful for implementing an IMAP server. For example, re-using the UniqueId (and related) classes for your IMAP server project would likely be useful. Likewise, if you eventually implement the THREAD extension, the MessageThreader class would be invaluable.
The ImapEncoding class would certainly be reusable.
You might be able to at least partially re-use the ImapStream (and ImapToken) class as your tokenizer for an IMAP server, but I'm not 100% sure on that since I've never looked at it from that perspective and there might be subtle differences there, but you could probably use it as a reasonable starting point.
ImapUtils.FormatInternalDate() would be a good candidate for re-use...
You might be able to reuse a lot of the FolderQuota and AccessControl classes as well if you end up supporting the QUOTA and/or ACL IMAP extensions (not that those classes do much).
I'm sure there are various other bits & pieces that would be helpful to re-use from MailKit, but you won't be able to easily turn the ImapClient class into an ImapServer class, for example.
And then, of course, as you mentioned in your post, you could always use MailKit as a great way of testing your IMAP server implementation to make sure things work.
Hope that helps answer your question.