Seeing "authentication failed", a 535 error, or an IMAP connection failure when connecting a mailbox to your sequencer? Work through these in order. (If instead the connection is refused, hangs, or never returns a banner at all — with no explicit auth error — see IMAP connects but times out, is refused, or returns no response instead, since that's a connectivity issue rather than a credentials one.)

If the mailbox was created recently and you're using the correct credentials, host and ports but it still returns an IMAP / authentication error, this is a known provisioning issue on our side — not something wrong with your setup. The fix is a quick IMAP-auth correction we run from the backend.

In this case, please don't keep recreating the mailboxes or re-exporting the CSV — it won't resolve it. Just contact support with the affected mailbox names and domains, we'll run the fix, and then you can reconnect (it usually connects right away afterward).

For Google Workspace mailboxes specifically, note they don't use the smtp.maildoso.com host at all — see Logging in to your Google Workspace mailboxes.

Our mail servers only authenticate mailboxes on domains with a valid, active DNS delegation. If a domain's status on the Domains page is NS Expired, NS Not Set, Inactive or suspended, every mailbox on it returns 535 on both SMTP and IMAP even though the credentials are correct. Fix the domain, not the credentials: for an external domain, regenerate the nameservers in the dashboard and set them at your registrar within 24 hours; for a suspended domain, contact us. Once the domain is Active again, export a fresh CSV and reconnect.

Registry suspensions of this kind are more common on some extensions (we see them most on .site, .online, .xyz and .biz when they're used for cold outreach); for new purchases .com, .co, .info and similar are the safer choice.

The same 535 pattern appears after a billing problem — see Renewing your domains and mailboxes.