Connect InboxKit Mailboxes to Lemlist in 5 Minutes

Lemlist accepts InboxKit Google Workspace and Microsoft 365 mailboxes two ways: Google/Outlook OAuth (1 click, required for Google-managed domains) and custom SMTP/IMAP (for any other provider or when OAuth is blocked). OAuth is the preferred path because Lemlist explicitly states that 'for Google-managed domains, all email addresses must use the API rather than SMTP.' Custom SMTP is the universal fallback and is the only option for Zoho, FastMail, Namecheap Private Email, or self-hosted mailboxes.

This guide walks through both paths with exact menu locations, the Google Workspace admin prerequisites that block most new connections, and the warmup settings Lemlist recommends before you launch a campaign.

Lemlist's own warning: 'If you're using a Google Workspace account, make sure your admin has enabled third-party app access, as without this Lemlist won't be able to connect to your email.' This is the single most common reason new connections fail. Source: Lemlist: connect your email provider.

InboxKit auto-provisions SPF, DKIM, DMARC, and MX records via Cloudflare in under 60 seconds at mailbox creation time, so the DNS row is already green by the time you reach Lemlist. For a refresher on the records that matter, see email authentication SPF/DKIM/DMARC explained.

Lemlist requires OAuth for all Google Workspace mailboxes that live on Google-managed MX records. InboxKit provisions mailboxes under Google-managed MX by default, so this is your primary path.

Total: about 100 seconds per mailbox.

The admin whitelist trap: If the OAuth pop-up shows 'This app is blocked by your organization,' the workspace admin must approve Lemlist in admin.google.com → Security → Access and data control → API controls → Manage Third-Party App Access → add Lemlist as trusted. Without this, the OAuth flow looks like it completes but the mailbox never appears as connected in Lemlist. Source: Lemlist: set up SMTP and IMAP.

Use this path only when the mailbox is not a Google Workspace or Outlook account: for example, a Zoho mailbox, a cPanel webmail mailbox, or a FastMail address proxied through an InboxKit domain. Google-managed Workspace accounts must use OAuth.

Source on port choice: 'SMTP Port is usually 587 (for TLS) or 465 (for SSL)', Lemlist SMTP/IMAP guide.

If your provider requires an app-specific password (Zoho, FastMail, some cPanel installs), generate it first in that provider's account settings before pasting into Lemlist. See the cold email sequencer integration guide for the full custom-SMTP reference.

Keep this next to your Lemlist setup screen. Google-managed accounts must use OAuth; the SMTP column is listed for completeness and only applies to non-Google-managed Gmail addresses.

Microsoft 365 specific: Microsoft disables SMTP AUTH by default on all new tenants since 2022. Your Exchange admin must enable it per-mailbox in Exchange Admin Center → Recipients → Mailboxes → (select) → Manage email apps → Authenticated SMTP. Without this, Lemlist returns 'Authentication unsuccessful, SmtpClientAuthentication is disabled for the Tenant.' This accounts for ~40% of Microsoft 365 + Lemlist failures. Source: Microsoft Authenticated SMTP.

Lemlist's warmup product is called Lemwarm. It is included with all paid plans (Email Pro and above). Turn it on before you send a single real campaign:

Lemwarm is included with all paid Lemlist plans (Email Pro and above). InboxKit's isolated warmup network at $3/mailbox/month is an alternative that runs in its own peer pool and does not share reputation with other Lemlist customers. Pick one; do not run both at once or you double warmup volume and confuse inbox classifiers. See cold email warmup guide for the full trade-off.

Google's hard ceiling for Workspace outbound relay is around 2,000 external recipients per day, but the practical cold-email ceiling is 40-50 per mailbox. To hit higher volumes, add more InboxKit mailboxes instead of pushing any single mailbox past 50/day. See email sending limits for Google and Microsoft.

Silent failure mode: Lemlist shows 'Connected' even when the OAuth scopes have been partially revoked. If sends suddenly stop and the mailbox still shows green, reauthenticate by removing and re-adding the sending address, Lemlist will force a fresh OAuth handshake.

Run these three tests before launching any campaign:

Test 1: Send a test preview. Settings → Sending settings → (mailbox) → Send test. Target your own inbox. Expect delivery within 60 seconds.

Test 2: Reply pulls into Lemlist. Reply to the test. Check the Lemlist inbox. If the reply shows within 2 minutes, IMAP is working.

Test 3: Lemwarm first-day activity. After 24 hours, check the Lemwarm dashboard. You should see ~10 sent, 5-8 received, 3-5 replies. If the counts are all zero, Lemwarm never started: most often because the daily limit was set to 0 and the warmup system interpreted that as 'paused'. For deeper inbox placement validation, see inbox placement testing explained.