How do the send queue and email backlog work?

Last updated: June 18, 2026

When a sequence has more sends pending than the mailbox can immediately deliver, emails wait in the queue. The Email Backlog dashboard is where you monitor and manage queued sends.

What's in the queue

Emails sit in Queued state for any of these reasons:

  • Scheduled future sends — emails set to send on a specific future date/time as part of the sequence cadence.

  • Daily-cap overflow — sends waiting because the mailbox has hit its daily send limit.

  • Outside-window sends — sends scheduled outside the sequence's send schedule window, waiting for the next valid window.

All three look the same in the UI but resolve differently — daily-cap overflow clears when the next day's window opens; future sends clear at their scheduled time; out-of-window sends clear at the next valid window.

Reassigning queued emails to a different mailbox

If a specific mailbox has a deep backlog and you want to route those queued sends through a different mailbox (to bypass the daily cap or distribute load):

  1. Go to Email Backlog in the dashboard.

  2. Scroll to Reassign Sequence Enrollments.

  3. Click Reassign Enrollments for the target sequence.

  4. Select the new mailbox(es) and save.

Important: this only works for queued enrollments. Emails already sent cannot be reassigned to a different sender. Emails currently in the process of sending (in flight) also can't be reassigned.

This is the primary tool for managing send capacity during high-volume launches.

Monitoring the backlog

Open Email Backlog to see:

  • Per-mailbox queue depth.

  • Distribution of sends across the next few days.

  • Any mailbox approaching its daily cap.

For high-volume campaigns, check this view daily during the launch period. Signals that indicate trouble:

  • A mailbox at or above its daily cap day after day — capacity isn't catching up. Add mailboxes or route to additional mailboxes.

  • Old queued enrollments not progressing — could be a stuck enrollment. Click into the enrollment to see what's blocking.

  • Sudden spikes in queue depth — usually means a big audience was just enrolled. Expected, but worth confirming nothing is misconfigured.

When queued emails aren't sending

If a contact is stuck in Queued and you don't see why:

  1. Is the sequence paused? A paused sequence accepts new enrollments but doesn't process the queue. Resume it.

  2. Is the mailbox at its daily cap? Check Email Backlog per-mailbox.

  3. Is the send schedule blocking? If "now" is outside the configured window, sends wait.

  4. Is the mailbox connected? A disconnected mailbox queues sends until it's reconnected.

  5. Is there an exclusion holding it? Exclusions are evaluated at send time, not just enrollment time. A contact who matched an exclusion mid-queue will be skipped at send time and shown as Excluded.

For deep-dive enrollment-side troubleshooting, see Troubleshooting sequence enrollments.

Best practices for high-volume launches

  • Route across multiple mailboxes from the start rather than concentrating on one.

  • Launch in batches — a few hundred contacts first, then waves of more. Monitor backlog between batches.

  • Set per-company enrollment caps (typically 2–4, max 7) to prevent over-concentration on individual accounts.

  • Watch the per-mailbox view daily during the launch.

  • Avoid reassigning between mailboxes mid-flight unless capacity demands it — repeated mailbox changes can affect deliverability.