Mailboxes & Warmup
Connecting a mailbox
- OAuth (Gmail/Google Workspace, Outlook/Microsoft) — no password ever touches Sendbox; you authorize through the provider’s own consent screen.
- SMTP/IMAP — for any other provider. Credentials are encrypted at rest and only ever used to send/check mail on your behalf.
Every mailbox belongs to a sending domain (derived from its email address), which is where deliverability is tracked.
Warmup
A new or long-dormant mailbox should warm up before real cold-email volume, so its sending reputation builds gradually instead of triggering spam filters on day one.
Starting warmup begins a ramp: a starting daily volume, a daily increase, and a target volume, tuned per mailbox provider type. Warmup activity is real send/reply/open traffic across Sendbox’s own warmup network — not synthetic pings.
A mailbox’s warmup stage — Week 1 → Week 2 → Week 3 → Steady State — advances automatically as it ramps. Only a mailbox at Steady State (or one you’ve explicitly marked as Sending Enabled) is eligible for cold campaign assignment.
Stopping warmup and restarting it later begins the ramp over from day 0 — there’s no “resume from where I left off.”
An address warms in one workspace only. If the same address is already warming in another workspace, starting or resuming its warmup here is refused with the reason. Resuming many mailboxes at once skips that one and resumes the rest.
Daily send caps
Every mailbox has a daily send cap (max 20/day, individually tunable) and every domain has a derived aggregate cap (the sum of its mailboxes’ caps, capped at 60/day) — Sendbox’s rate limiter enforces both.
Health status
Sendbox tracks bounce rate per mailbox on a rolling 24-hour window and auto-pauses a mailbox whose bounce rate crosses an unsafe threshold, protecting the rest of your sending reputation.
Mailbox problems
Sendbox watches for three problems that stop a mailbox from doing its job:
| Problem | What it means |
|---|---|
| Login refused | Its SMTP login is refused (for example a wrong or revoked app password). Nothing sends from it, campaign or warmup |
| Replies not syncing | Its replies could not be read for 30 minutes or more. Replies stop reaching your inbox |
| Paused for bounces | It was paused automatically because too many of its emails bounced |
When a mailbox has a problem:
- The Mailboxes menu item shows a red count.
- The Mailboxes page lists each problem, since when, and what to do.
- Owners and admins get an email. Each problem is emailed once, and at most one email goes out every 10 minutes.
A mailbox you paused yourself reports no problem.
Signatures & tags
Set a per-mailbox HTML signature, and attach tags for organization or for use in a campaign’s tag-based mailbox pool selection.
Test-send
Use Send test email to confirm a mailbox’s credentials actually work, without building a whole campaign.