Use 3-5 mailboxes per domain, with each mailbox sending 30-50 emails daily. For 500 emails per day, you need approximately 10-15 mailboxes distributed across 2-5 domains. This mimics natural human sending patterns and maintains strong deliverability.
The Mailbox Math
Email service providers analyze sending behavior at the mailbox level. A single mailbox sending hundreds of emails daily looks automated—because it is. Real humans send 20-50 emails per day. Your cold email infrastructure should mirror this pattern.
SendroAI automatically distributes your campaigns across multiple mailboxes, ensuring each one stays within safe sending limits while maximizing your total daily volume.
Mailbox Volume Guidelines
Use these thresholds when planning your mailbox infrastructure:
- New mailboxes (weeks 1-2): 10-20 emails/day maximum
- Warming mailboxes (weeks 3-6): 20-40 emails/day
- Established mailboxes (6+ weeks): 40-75 emails/day
- Maximum safe limit: 100 emails/day per mailbox
These limits apply to total sends, including follow-ups. If you're running multi-touch sequences, account for follow-up volume in your calculations.
Calculating Your Mailbox Needs
Here's a practical formula for determining mailbox requirements:
- 100 emails/day: 3-4 mailboxes across 1-2 domains
- 250 emails/day: 6-8 mailboxes across 2-3 domains
- 500 emails/day: 12-15 mailboxes across 3-5 domains
- 1,000 emails/day: 25-30 mailboxes across 5-10 domains
Always maintain buffer capacity. If a mailbox develops reputation issues, you can redistribute volume to healthy mailboxes without disrupting campaigns.
Mailbox Naming and Personas
Each mailbox should represent a believable sender identity. Use realistic first and last name combinations:
- sarah.johnson@yourdomain.com
- mike.chen@yourdomain.com
- emma.williams@yourdomain.com
Avoid generic addresses like sales@, info@, or outreach@. These immediately signal automated sending and reduce open rates. Personal names build trust and encourage engagement.
The Domain-Mailbox Relationship
Understanding how domains and mailboxes interact is crucial. Each domain can host multiple mailboxes, but all mailboxes under a domain share that domain's reputation. If the domain reputation suffers, all associated mailboxes are affected.
This is why SendroAI monitors both mailbox-level and domain-level reputation metrics, automatically throttling or pausing sends when warning signs appear.
Warm-Up Requirements
Each new mailbox requires individual warm-up, regardless of domain age. Even on an established domain, a new mailbox starts with zero sending history. Plan your infrastructure buildout to account for warm-up timelines—you can't immediately deploy 15 mailboxes at full volume.
SendroAI includes automated warm-up protocols that gradually increase sending volume for each mailbox while maintaining engagement rates that build positive reputation signals.
Team Considerations
For sales teams, consider assigning specific mailboxes to individual reps. This personalizes outreach and ensures consistent sender identity throughout the prospect journey. However, the platform should still manage volume distribution to prevent any rep from overloading their assigned mailboxes.
Key Takeaways
- 30-50 emails per mailbox per day is the safe zone
- Plan 3-5 mailboxes per domain for flexibility
- Use realistic personal names, not generic addresses
- Each mailbox needs individual warm-up
- Maintain buffer capacity for redistribution