Bulk Email Warm-Up: Warm 50 to 1,000 Mailboxes From One CSV

For 50 to 1,000 mailboxes, budget $120/month to $1,980/month on a new BobWork account after the 10 free mailboxes. If you connect them one by one, setup errors and uneven ramps multiply fast. This guide gives you the CSV upload path, ramp maths, pool choice, domain spread rules and readiness checks.

If you want a smaller walkthrough first, start with the free warm-up tool or the 50-mailbox bulk guide. If you already know your fleet size, jump to the matching page: 100 mailboxes, 200 mailboxes, 300 mailboxes, 500 mailboxes or 1,000 mailboxes.

What does bulk email warm up cost?

Bulk warm-up pricing only makes sense when you price the whole fleet. A small per-mailbox difference becomes large at 500 or 1,000 mailboxes.

BobWork Email Warm-up is free for up to 10 mailboxes on new accounts. Mailboxes above the free limit are billed monthly. Early users who created accounts before 27 October 2026 keep 20 free mailboxes for as long as they use the product.

Prices below are for new accounts with 10 free mailboxes. Extras pause without billing when inactive. Billing is handled by Stripe, prorated and can be cancelled any time.

The default account limit is 200 mailboxes. Fleets above that, including 500 and 1,000 mailboxes, can request a higher limit through info@bobwork.ai.

Use the fleet pages when you need the full plan for a specific size. The 200-mailbox guide covers the default account limit. The 500-mailbox guide and 1,000-mailbox guide focus on higher-limit fleets.

How do you upload the mailboxes?

Do not connect hundreds of inboxes by hand. Use the bulk CSV route.

In BobWork, go to Add mailbox → Many mailboxes: upload a CSV. Paste rows or upload a file with these columns:

Gmail, Yahoo and iCloud hosts are filled in automatically. Other providers need the IMAP host in the file.

For Gmail and Google Workspace, the recommended route is IMAP with an app password. App passwords require 2-Step Verification. Google’s own help explains how to create and manage app passwords.

Google one-click sign-in also exists, but it is limited. Google caps unverified apps at 100 users. BobWork has not done the CASA audit, so Google shows an unverified-app notice and spots are limited. A Workspace admin can mark the app as trusted in the Admin console.

Every CSV row gets the same IMAP and SMTP login test as a single mailbox connection. Failed rows are listed with the reason. Already-connected rows are skipped, so re-uploading the same CSV is safe.

Mailboxes above the free limit need billing active before they connect. That matters when you upload 100 rows into a new account. The first 10 can fit the free allowance. The rest need active billing.

If you are connecting many Gmail or Google Workspace accounts, use the bulk Gmail accounts guide. For the file format itself, use the CSV import guide.

How fast will the fleet ramp?

Each mailbox ramps by itself. That is the main rule for fleet planning.

A mailbox starts at 3 warm-up emails per day. It then adds 2 per day until it reaches 30 per day. The cap is editable per mailbox, up to 100. Weekends run at 30% of the current target.

Sends are spread between 09:00 and 18:00 in the mailbox’s own time zone. The engine decides every 15 minutes with randomness. A pair never writes to each other twice within 3 hours. Peers on a different domain or provider are picked twice as often.

Here is the weekday ramp for common fleet sizes. Weekend days are 30% of the relevant day’s target.

These are warm-up emails, not cold emails. Do not treat a ramped 500-mailbox fleet as permission to send 15,000 cold emails per day.

For cold outreach, keep real sending conservative. A typical practice is 30–50 cold emails per day per mailbox. Add mailboxes instead of forcing more volume through each one.

BobWork’s published practice is to warm an existing domain for 7 days and a new domain for 2–3 weeks. During active campaigns, keep warm-up at 10–15 per day per mailbox. The warm-up schedule guide shows the day-by-day version.

Which pool should you choose?

BobWork uses a shared warm-up network by default. Every active mailbox on the service exchanges warm-up emails with other active mailboxes across accounts. One mailbox is enough to start because it can warm with the network.

A private pool is optional. Switch it in Mailbox pool → Pool mode. In private mode, only your own mailboxes write to each other. Nobody outside your account receives those addresses. A private pool needs at least two mailboxes.

For 50+ mailboxes, a private pool is usually the cleaner choice. You already have enough volume inside the account, especially when the mailboxes span several domains or providers.

The free shared pool does the matching automatically. Users never write the text. Warm-up messages are short plain-text office notes with no links or images. Threads end after 3 messages, and about 40% of received warm-up mail gets a reply.

Warm-up mail found in Spam is moved to Inbox, marked read, labelled “Warm-up”, and archived after 24 hours. Only mail with the engine’s hidden header is touched. For Zoho API senders, a known subject is used. Real mail is never read, moved or answered.

If you are comparing this with manual sending, read manual versus automated warm-up. Manual work breaks down fast once you manage dozens of mailboxes.

How should domains be spread?

The fastest way to burn a fleet is to treat all mailboxes as one large pipe. A healthier setup spreads mailboxes across several domains.

For big fleets, a typical cold email setup uses 2–5 mailboxes per domain. That means 100 mailboxes should usually sit across many domains, not one. The exact split depends on your risk tolerance, market, offer and sending plan.

Every sending domain needs authentication. Do not warm hundreds of mailboxes before DNS is correct.

At minimum, check these records on every domain:

Google’s email sender guidelines tell senders to keep the spam complaint rate under 0.1% and avoid ever reaching 0.3%. Yahoo’s sender guidance requires SPF and DKIM, DMARC with passing alignment, and a spam rate under 0.3% for bulk senders. Use the primary guidance from Google and Yahoo when you set policy.

If you use one-click unsubscribe for cold campaigns, follow the header standard in RFC 8058. Warm-up is not a substitute for consent, suppression lists or clean unsubscribe handling.

BobWork checks SPF, DKIM and DMARC daily and shows the result per domain. It looks for common DKIM selectors including google, zmail, default, selector1, selector2, k1, s1, mail, dkim, lark, feishu and zoho.

Use the free SPF checker, DKIM checker and DMARC checker before uploading a large CSV. Fix DNS first, then connect mailboxes.

How do you read results?

Fleet averages can hide bad mailboxes. Read results at mailbox level and domain level.

BobWork’s spam rate is the share of a sender’s warm-up mail that receiving mailboxes found in Spam over the last 7 days. It is measured directly inside receiving inboxes, not through third-party seed lists.

The health score runs from 0 to 100:

A mailbox is ready for outbound only when all three checks pass: at least 7 days warming, 7-day spam rate of 3% or lower, and at least 30 warm-up emails received.

Do not average your way past that gate. If 190 mailboxes are healthy and 10 are at risk, send only from the healthy ones. Pause any mailbox above 5% spam rate for a few days.

When a whole domain performs badly, inspect the domain first. Check DNS, recent cold volume, content, bounce rate, complaint risk and provider blocks. If only one mailbox is bad, inspect that mailbox’s login status and recent sending pattern.

For new domains, use the new domain warm-up guide. For the difference between domain and mailbox reputation, read domain warm-up versus mailbox warm-up.

What should you do now?

Bulk warm-up works when setup is boring. The same records are correct everywhere. The same CSV columns are clean. The same readiness rule applies to every mailbox.

Use this checklist before you upload hundreds of accounts.

If you need the shortest path, create your CSV, connect the first batch, fix failed rows, then switch to a private pool once the fleet is large enough. Keep warm-up running while you send, but do not let cold email volume outrun the readiness data.

BobWork Email Warm-up can handle the bulk path directly: CSV upload, shared warm-up network or private pool per account, per-mailbox ramping, daily DNS checks and mailbox-level readiness. Start with the free warm-up tool, then use the fleet guide that matches your size before you upload the full file.

Frequently asked questions