mailkit is multi-account. Each account has one provider. Unsupported verbs error. They never silently succeed.
Verbgmailfastmailprotonmail
auth *, read, search, showyesyesyes
draft, reply, drafts, sendyesnoyes
archive, mark-read, mark-unread, trashyesnoyes
thread, label, labels, labels-createyesnono
untrashyesnono
filter *, reconcile, file-inboxyesnono

​
Gmail

Full surface. Message ids and thread ids are Gmail ids. Search is Gmail search syntax.

​
Fastmail

Read path only. Credential is a static API token. Search compiles only:
  • from:x
  • from:(a.com OR b.com)
  • -from:x
  • in:inbox
Anything else raises rather than matching a silent subset.

​
Proton Mail

Read plus send/draft through Bridge on this Mac. Message ids are mailbox:uid, for example INBOX:12. Use that same string with show, reply, archive, trash. Search uses the same small from: / in: subset as Fastmail. Bridge must stay running. See Setup.

​
If you guess wrong

error: 'send' is not supported on fastmail accounts (you@fastmail.com).
Check mailkit auth list and pick a Gmail or Proton Mail account for write verbs.