Split a list across inboxes
How it works
Load the list
Upload a CSV or paste the rows. The email column is found by its header, or by looking for the column that contains addresses if there is no header.
Clean it
Duplicate addresses, malformed rows, free mailbox domains and optionally role addresses come out first, then the per-company cap trims the rest. Every removal is counted so you can see what the source really contained.
Assign inboxes
Round robin balances by size; blocks keep the original order in contiguous chunks. With company grouping on, every contact at a domain lands on the same inbox regardless of which mode you pick.
Get the schedule
Each row gets a send day from your per-inbox daily cap and ramp setting. Download the master CSV with the two new columns, or one file per inbox to upload separately.
The problem with one big upload
Sequencers rotate inboxes, but most rotate by message rather than by account, and none of them know that jane@acme.com and tom@acme.com sit ten feet apart. When both get the same opening line on the same morning from two different senders at your company, the sequence is discovered in one forwarded email. That is a reply-rate problem before it is a deliverability problem, and no amount of warmup fixes it.
Splitting deliberately solves three things at once: it caps how many people at one company hear from you, it guarantees they hear from one sender, and it turns an abstract list size into a real number of days at your actual sending capacity.
Round robin or blocks
Round robin interleaves, so every inbox gets a slice of the whole list and quality is spread evenly. If the list is sorted by anything meaningful — company size, score, geography — that matters, because block mode would hand inbox 1 all the best rows and inbox 10 the tail. Blocks are useful when the order is the point: a list already sequenced by priority, or one where you want to run the first chunk and hold the rest.
Company grouping sits above both. Turn it on and the splitter assigns whole companies to the emptiest inbox rather than assigning rows, which keeps the counts close to even without ever splitting a company. The only reason to turn it off is when the list is one contact per company anyway.
Caps, ramps and what the schedule means
The daily cap is per inbox, and it should be the number you actually send, not the number the platform allows. Thirty a day from a mature Google Workspace inbox is comfortable; fifty is the top of the range that still looks human; the ramp settings exist because a domain that has been sending nothing and starts at thirty is a pattern every filter knows. Gentle starts at ten and adds five a day. New domains starts at five and adds three. Both stop at your cap.
The last-day number that comes back is capacity arithmetic, not a forecast. It assumes every inbox is authenticated, warm and not on a blacklist. Check that separately — the capacity calculator for how many inboxes a target volume actually needs, the warmup calculator for how long the ramp should be, and the deliverability score for whether the domains are ready at all.
What gets removed, and why
Free mailbox domains are off by default in B2B lists because a consumer Gmail address in a scraped B2B export is usually noise, and consumer recipients hit the spam button far more readily than someone reading mail at work. Role addresses are kept by default, because in small businesses info@ is the owner's inbox and the right target — but on an enterprise list they are a shared mailbox nobody owns, so turn the filter on. The per-company cap defaults to two. Three is defensible if the sequences genuinely differ by role. Five is a blast.
Run the output through the email verifier or send it to Rooster4am for a full clean before the first send — splitting a list that is 20% invalid just distributes the bounces evenly across your fleet.
Frequently asked questions
Does my list get uploaded anywhere?
No. The file is read by your browser with the local file API, split in the page, and the CSVs are generated on your device. Nothing is sent to a server.
How large a file can it handle?
A few megabytes comfortably — tens of thousands of rows. Very large files will make the browser think for a moment; split them first if the tab stalls.
What if my CSV has no header row?
It still works. The first column containing an address is treated as the email column and the others are named col_1, col_2 and so on in the export.
Why is the per-company cap applied before the split?
Because capping after assignment would leave uneven inboxes. The order is dedupe, filter, cap, then assign.
Can I get one file per inbox?
Yes — each row in the inbox table has its own CSV button, and the master file carries the assigned_inbox column if your sequencer can filter on it.
Does send_day account for weekends?
No. It counts sending days. If you only send Monday to Friday, multiply by seven fifths for the calendar date.
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The diagnostics here are free and always will be. When the fix is bigger than a DNS record, this is the work I do.
Deliverability rescue
Mail landing in spam, replies gone quiet, or a domain suddenly blocked. I find the actual cause rather than guessing, and fix it.
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- Domain and inbox fleets at any scale
- SPF, DKIM, DMARC and tracking domains
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Domain, DNS & migration
Changing registrar, mail provider or host without a day of downtime or a week of mail silently failing.
- Registrar and nameserver moves
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- MX, SSL and subdomain cutover
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Monitoring & retainer
Infrastructure drifts. Records get edited, certificates expire, domains get listed. Ongoing eyes on the fleet.
- Scheduled checks across every domain
- Alerts before your clients notice
- Monthly reporting
- Priority response when something breaks
Start with a call
Bring a domain and the symptom. I will tell you what is actually wrong and whether you need me at all — plenty of people leave that call able to fix it themselves.
Thirty minutes, no pitch
We will run the checks together on your actual domains, and you will leave knowing what is broken, what it takes to fix, and what it should cost. If that is a job you can do in-house, I will say so.