# How Reply selects a sending mailbox

> The mailbox assignment rule — round-robin distribution across up to 100 accounts, follow-up thread continuity, provider matching, and reassignment on removal.

**Reply assigns each new contact to one of up to 100 sequence email accounts using round-robin logic, from any source. All follow-ups send from the same account as the first email unless that account is removed. Matching email providers, when enabled, pairs contacts with same-provider senders; the Reassign email accounts setting redistributes contacts automatically.**

_Status: Reviewed — human-edited, facts not yet confirmed against the product._

## At a glance

| Aspect | Detail |
| --- | --- |
| **What it governs** | Which of a sequence's email accounts is assigned to each contact, and which account each email sends from |
| **Trigger** | A contact is added to a sequence with connected email accounts (manually, via CSV, API, or integrations) |
| **Inputs** | Email accounts added to the sequence, contact's detected email provider, Matching email providers toggle, Reassign email accounts setting |
| **Result** | The contact gets a sending email account; the first email and all follow-ups go from it, in one thread |
| **Configurable** | Sequence email accounts, Matching email providers (per sequence), Reassign email accounts (account-level setting) |
| **Does not** | Decide when the send happens — that is [When Reply sends](/behavior/sending-schedules) — or reassign contacts already assigned to an active account |
| **Plans** | Adding teammates' email accounts to one sequence requires Team Edition |

## Rule summary

A sequence can send from **up to 100 email accounts**. New contacts — added manually, via
CSV, API, or integrations — are assigned to the connected accounts **one by one using
round-robin logic**: the first contact to the first account, the second to the second, and
so on, cycling back to the first. If the sequence has multiple email steps, **all
follow-up emails for a contact are sent from the same email account as the first email**,
unless that account is removed from the sequence.

## Trigger

A contact is added to a sequence that has one or more email accounts connected, or the
sequence's email accounts change (an account is added or removed).

## Decision logic

1. When a new contact enters the sequence, Reply picks the next email account in round-robin order, spreading sending evenly across all connected accounts.
2. If the **Matching email providers** toggle is on for the sequence, Reply first pairs the contact with a sending account whose email provider matches the contact's (Outlook to Outlook, Gmail to Gmail). Unmatched contacts are distributed among the available accounts using round-robin logic.
3. The assigned account sends the contact's first email, and every follow-up sends from that same account — the conversation stays in one thread.
4. If the contact's account is removed from the sequence, follow-ups send from the account the contact is reassigned to, still in the same email thread.

**Assignment when accounts change:**

| Action | Condition | Assignment behavior |
| --- | --- | --- |
| Connect a new email account | No other account connected, or contacts have never had accounts assigned | Reply assigns the new sending email automatically |
| Connect a new email account | Active accounts exist and contacts are assigned to them | Nothing changes for assigned contacts; newly added contacts are assigned across all accounts, including the new one |
| Upload contacts to a sequence | One or more accounts connected | New contacts are assigned automatically across all connected accounts |

## Result

Each contact shows a sending email account (the **Sending email** column in the People
tab). The first-step email and all follow-ups for that contact go out from it.

## State changes

| Field | Change |
| --- | --- |
| Contact's sending email | Set on assignment; changes only on removal, reassignment, or a manual change |
| Email thread | Preserved — even after switching accounts, new emails arrive in the same thread |
| Sequence toggle | An error notification appears over the toggle when contacts are left unassigned |

## Exceptions

- **Removing an account leaves its contacts unassigned** — you must assign them manually (filter People by Unassigned, then link a sending email) unless the **Reassign email accounts** setting is enabled.
- **Manual assignment is also needed** when you add a new account to replace a deleted one, or when contacts are already assigned to existing accounts and you expect them to move — Reply does not change existing assignments automatically.
- **Provider detection happens only in sequences** — a contact merely uploaded to Reply but not added to a sequence gets no provider detection; undetected providers show as `Unknown`.
- **Toggling Matching email providers** on or off does not touch existing assignments — only newly added contacts follow the new logic.
- **Teammate signatures** — if a removed account's contacts are reassigned to a teammate's account with a different signature, update the follow-up templates so the thread reads consistently.

## Configuration that changes this rule

**Matching email providers** (Sequence > Settings > General settings, off by default)
switches assignment from pure round-robin to provider-matched first. **Reassign email
accounts** (Settings > Emails > Sequence email accounts) changes removal behavior: only
contacts without a sending account are auto-assigned, contacts added before any account
was connected get assigned once mailboxes are added, and contacts of a removed account are
redistributed among the remaining accounts without touching other assignments. You can
also change a contact's sending email manually from the sequence People tab — sequence
owners only, and the new account must already be in the sequence's Settings tab. Also
raise **Max new daily contacts** and **Max number of emails per day** to match the number
of accounts added — see [Configure sending limits](/how-to/configure-sending-limits).

## Limits

| Limit | Value |
| --- | --- |
| Email accounts per sequence | Up to 100 |
| Assignment order | Round-robin, one contact per account in turn |
| Follow-up account | Same as the first email's account, unless that account is removed |
| Who can change a contact's sending email | Sequence owners only |

## FAQ

### Does a contact always hear from the same mailbox?

Yes — all follow-ups send from the same account as the first email. The only exception is
removing that account from the sequence: the contact's follow-ups then send from the
reassigned account, still in the same thread. See [Email accounts](/specifications/email-accounts).

### Why are contacts not assigned to my email?

Three common causes: an active sending email was deleted (its contacts are not reassigned
automatically), a new account was added to replace a deleted one, or a new mailbox was
added while contacts were already assigned to existing ones. Filter the sequence People
tab for Unassigned contacts and link a sending email, or enable Reassign email accounts.
See [Why is a campaign not running?](/troubleshooting/campaign-is-not-running).

### What happens if I switch Matching email providers mid-sequence?

Nothing for contacts already assigned — the toggle affects only newly added contacts.
Matched contacts get a same-provider sender; unmatched ones fall back to round-robin.

### Can I send one sequence from my teammates' mailboxes?

Yes — with Team Edition enabled you can add both your own and teammates' email accounts to
the same sequence.

## Related

- [Email accounts](/specifications/email-accounts)
- [Sequence execution](/behavior/sequence-execution)
- [When Reply sends](/behavior/sending-schedules)
- [Configure sending limits](/how-to/configure-sending-limits)

## Build with Reply

- REST API: [docs.reply.io](https://docs.reply.io/api-reference/introduction) — sequence account assignment endpoints
- MCP: [agents.reply.io/mcp](https://agents.reply.io/mcp) — agents assign accounts to sequences