If you see "Provider 'maildoso' is temporarily not supported" when trying to sync your mailboxes to Instantly, Smartlead, or another sequencer with the one-click connection, this isn't an error on your account — one-click sync only ever works for Google Workspace mailboxes. SMTP (Maildoso-provider) mailboxes are not supported for one-click sequencer sync, by design — this isn't a temporary gap, and it applies regardless of which sequencer you're connecting to.
What to do instead
Connect your SMTP mailboxes manually using CSV instead of the one-click sync:
Go to Email Accounts in your Maildoso dashboard and choose Export CSV to download your mailbox credentials.
Import that CSV directly into your sequencer (Instantly, Smartlead, etc.) — most tools have a "bulk import" or "add via CSV" option alongside the one-click connect button.
This avoids the "temporarily not supported" error entirely and also prevents the connection/verification errors that can happen when a sequencer tries to auto-sync a large batch of mailboxes at once.
Will one-click sync ever support SMTP mailboxes?
Not currently planned — one-click sequencer sync is built specifically for Google Workspace mailboxes, and SMTP mailboxes are expected to stay on the CSV/import path. The CSV method above uses the same underlying data, it's just added manually rather than through the one-click connector. See also How do I upload emails to my cold email tool? and Connecting your mailboxes to Instantly.