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 (see Connecting Google Workspace mailboxes to Smartlead, Instantly, and other sequencers). 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, select the mailboxes you want to export, 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.
Specific errors to watch for during CSV import include:
422 Error: IMAP Validation Failure: Retry with a fresh CSV file and wait briefly before re-importing.
409 Error: Mailbox Already Exists: Check for duplicates or disconnected entries, remove them, and retry the import.
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.
For advanced users, Maildoso also provides an API for programmatically exporting mailbox credentials using a Personal Access Token. This can be integrated with Instantly or Smartlead’s APIs for automated imports.