How to Split a Lead List Across Your Sending Inboxes (Even Shares, Caps and Same-Company Rules)

To split a lead list across sending inboxes, add one column that names the inbox for each row, then load each inbox's rows into its own campaign. That is the whole job, and it is easy to do badly by hand: sorting in a spreadsheet, cutting and pasting blocks, and discovering later that six people from one company all landed on different inboxes. This post is the method, the choices you need to make first, and a free tool that does the assignment.

Split a list across your inboxes →

Why split a list at all

A single inbox can only send so much in a day. Providers publish their own limits, for example for Google Workspace and Exchange Online, and your sequencer or your own caution may set a lower one. If one list has to go out in a reasonable time, several inboxes have to share it. Our post on inbox rotation covers what rotating does and does not spread, and how many cold emails per day each inbox should send covers the daily number.

What those posts do not cover is the mechanical step in between: you have one file of leads and a handful of inbox addresses, and every row needs an owner. Some sequencers rotate for you. Many setups, particularly several separate campaigns or separate accounts, need the list divided before upload.

Step 1: Decide the company rule

Before assigning anything, choose how people at the same company are handled. Companies are found from the email domain, and personal mailboxes such as Gmail or Outlook are never treated as one company, because two people at gmail.com have nothing in common.

Keep together. Everyone at one company goes to one inbox. The company is contacted from one address. This is the simpler story for the recipient: one sender, one thread of context. The cost is lumpy shares, because a large company is a big block that has to sit on one inbox.

Spread apart. Each lead goes to the lightest inbox that has not yet been given a lead at that company, so colleagues hear from different addresses. Shares come out more even, and a company that blocks or flags one of your senders does not take all your leads at that company with it. The cost is that a company can receive mail from several of your addresses, which some teams do not want.

Neither is correct for everyone. If you sell to small companies with one buyer each, the choice barely matters. If you sell into large accounts, think about it before you start. If you also want the colleagues at one company separated in time, the same-domain send spacer reorders rows for that, and capping the number of contacts per company keeps one account from dominating the list.

Step 2: Decide the cap per inbox

Equal shares are the default: every inbox gets the same number of leads, give or take one. But your inboxes may not be equal. A newer inbox with little sending history may deserve fewer leads than one that has been in use for months, and some inboxes may be reserved for follow-ups.

A cap sets the most leads one inbox can take from this file. The number is yours, and it depends on three things:

• how many emails each inbox sends a day,
• how many days the campaign runs,
• what your mail provider allows.

Note that a cap on leads is not a cap on emails per day. A lead in a three-step sequence receives three emails, so a cap of 600 leads is a lot more than 600 sends. If you are not sure how to turn leads into a daily plan, the email send schedule planner and the cold email volume calculator work from the other direction.

When the list is larger than the caps allow, the extra rows are left without an inbox. That is useful information: it tells you that you need more inboxes, or a longer campaign, or a smaller list. Add inboxes and run the split again.

Step 3: Run the split

With the lead to inbox assigner:

1. Upload the lead list (.xlsx or CSV), or paste the cells with the header row first. The email column is found for you.
2. Paste your sending inboxes, one per line, and choose keep together or spread.
3. Set a cap per inbox if you have one, then read the table of leads per inbox.
4. Download the CSV.

The tool adds a column called Sender inbox. It does not send anything and does not connect to your mailboxes; your inbox addresses are only used as labels. The file is read and rewritten in your browser, so the list is not uploaded to us.

Step 4: Cut the file into campaigns

Open the downloaded CSV and filter by Sender inbox. Each filtered block is the list for one campaign, or one account in your sequencer. Save each as its own CSV and upload it where that inbox lives. If your sequencer has its own import quirks, we have walkthroughs for Instantly, Smartlead and lemlist.

If your sequencer has a built-in rotation across connected inboxes, you may not need the split at all. The assigner is for the cases where you want control over which lead goes to which inbox, or where inboxes sit in separate tools.

Step 5: Verify before you split, not after

Order matters. If you split first and verify afterwards, you have to verify several files and re-balance the shares when dead addresses are removed from some blocks and not others. If you verify first, you remove the dead rows once, and the split then divides only addresses that exist.

There is also a deliverability reason. A dead address bounces from whichever inbox it was given. Bounces on a mailbox count against that mailbox, so a block of bad rows given to one inbox can hurt that inbox's record while the others stay clean. Verifying the list first keeps the bounces out of every inbox. See the bounce budget per inbox for how to think about the number you can afford, and adding new inboxes to a live campaign if you grow the pool mid-send.

Upload the list at /verify and keep the valid rows. For catch-all and unknown results, decide what goes into the send before you split; the catch-all send limit calculator and what to do with catch-all emails cover that choice.

What the assigner does not know

Be clear about the limits so you use the output properly:

• The cap counts leads for the whole file, not emails per day. Spread over days with a schedule.
• It does not know which inboxes are warm or which provider each one is on. Give it only the inboxes that should send.
• Equal shares are not always the right shares. Use a cap to correct for that.
• Company is judged by the email domain, so two domains of one company count as two companies.

A worked example, in words

Say you have a file of leads and four inboxes. You choose spread mode, with no cap. Each inbox gets a quarter of the rows, give or take one. Where a company has three leads, they go to three different inboxes. Where a company has five leads and you only have four inboxes, at least two of its leads must share one, and the tool places them on the lightest inbox. Now say one of the four inboxes is new and you cap it lower than the others. The cap is reached, the remaining rows are spread across the other three, and anything over every cap is left without an inbox, counted in the results.

We are deliberately not quoting numbers for how many leads an inbox should take, because that depends on your provider, your sequence length and your sending history.

Where verification fits on price

The assigner is free. Verification is the paid step, and it is the one that protects each inbox. SimpleVerifier Pro is $39 a month, with the first 100,000 checks a month at full speed, then slower, never cut off. For the whole range, see pricing. Most lists are verified once up front, then again when they age; how often to verify a list covers the timing.

The short version

1. Verify the list and remove dead rows.
2. Choose keep-together or spread for companies.
3. Set a cap per inbox if your inboxes are not equal.
4. Run the lead to inbox assigner.
5. Filter by Sender inbox and load each block into its campaign.
6. Watch the bounce rate per inbox, not just overall.

Verify your list before you split it →