Mailboxes

10 min read Last updated: June 23, 2026

Mailboxes in ProperSend

Each cold email campaign sends messages through configured SMTP mailboxes. A mailbox in ProperSend includes full SMTP (sending) + IMAP (receiving) credentials for your domains, plus automatic warmup and reputation monitoring. All replies are routed to Unibox via IMAP.

When to set up mailboxes: Before launching any campaign. You need at least one properly configured mailbox to send emails. For best deliverability, set up 3-5 mailboxes on different domains. This distributes sending volume and protects your primary domain's reputation.

Navigate to app.propersend.pl/mailboxes to manage your mailboxes.

Adding a Mailbox

  1. Click Add Mailbox in the top-right corner
  2. Select a provider from the preset list (Google Workspace, Outlook, etc.) or configure manually with custom SMTP settings
  3. Enter your SMTP server, port, username, and password — plus IMAP settings for reply monitoring
  4. Click Test Connection to verify credentials are correct
  5. Save — the mailbox appears in your list and starts the warmup process

Tip: Always test the connection after adding a mailbox. A failed test usually means incorrect port (use 587 with TLS for SMTP, 993 with SSL for IMAP) or an app password required by your provider (Google and Outlook require app passwords when 2FA is enabled).

IMAP for Reply Monitoring

ProperSend uses IMAP to monitor replies from your leads. When a lead responds, the message is fetched via IMAP, classified by AI, and appears in Unibox. This means:

  • You don't need to check each mailbox separately — all replies land in Unibox
  • AI automatically classifies each reply (interested, bounce, unsubscribe, OOO)
  • You can respond directly from Unibox using the same sending domain

If IMAP sync stops working, check your mailbox credentials and ensure IMAP is enabled in your provider's settings. See Troubleshooting for common issues.

Domain Rotation

To protect your deliverability and avoid blocks, ProperSend supports domain rotation. Add multiple domains to a mailbox pool and the system automatically selects one for each send. This distributes volume across domains, so no single domain gets flagged for high sending frequency.

Practical example: Instead of sending 200 emails/day from hello@company.com (risky), set up 5 mailboxes on different domains (@company1.com, @company2.com, etc.), each sending 40 emails/day. The sending load is distributed, and if one domain gets flagged, the others keep running.

Warmup

Warmup is the process of gradually increasing daily sending volume to build domain reputation with email providers (Google, Outlook, etc.). ProperSend automates this by sending and receiving warmup emails from a network of trusted mailboxes.

| Week | Daily limit | Hourly limit | |------|-------------|--------------| | 1 | 5-10 | 2-3 | | 2 | 15-25 | 4-5 | | 3 | 30-50 | 8-10 | | 4 | 50-100+ | 15-20 |

💡 Tip: Don't skip warmup. Sending 100+ emails/day from a brand-new domain is a fast track to the spam folder. Even with a warm domain, start conservatively when using it for cold email. Complete warmup before launching full-volume campaigns.

The warmup process runs automatically — no manual action needed beyond connecting the mailbox. You can monitor progress on the mailbox details page.

Reputation

ProperSend continuously monitors each mailbox's health metrics (30-day rolling window):

  • Inbox rate – percentage of emails landing in the primary inbox (vs. spam)
  • Spam rate – percentage marked as spam (target: <0.1%)
  • Bounce rate – percentage of emails that bounced (target: <3%, auto-pause at 5%)
  • Volume sent (30d) – total emails sent over the last 30 days

⚠️ Note: If spam rate exceeds 0.1% or bounce rate exceeds 5%, reputation drops to "low" and campaigns using this mailbox may be auto-paused. Review the Domain Health dashboard and verify your Suppressions list. Configure alert thresholds in Notifications.

Mailbox Groups

Organize mailboxes into groups for easier campaign assignment. Each campaign can be assigned to a specific group, ensuring leads are only contacted from mailboxes in that group. This is useful for:

  • Agency use — separate client campaigns into different mailbox groups
  • Geographic targeting — use Polish domains for Polish campaigns, EU domains for EU campaigns
  • Brand separation — keep different brands/products in isolated mailbox pools

Best Practices

  1. Use 3-5 mailboxes on different domains for a single campaign
  2. Complete warmup before running full-volume campaigns
  3. Monitor reputation weekly — check Domain Health and mailbox stats
  4. Set up SPF, DKIM, and DMARC for every sending domain (see Domain Health guide)
  5. Remove under-performing mailboxes — if one consistently shows high spam rates, pause it and investigate
  6. Use mailbox groups for multi-client or multi-brand setups
  7. Test IMAP connectivity after adding mailboxes — replies are useless if IMAP isn't working