High density looks efficient on a spreadsheet. Then one sender goes sideways and exposes every other sender sharing the domain to the same reputation problem.
When agencies size outbound capacity, the instinct is to pack as many inboxes onto each domain as possible. Fewer domains, less DNS to manage, one bill. On paper it is efficient. In practice it concentrates more sending activity on each domain, one of the main reputation-bearing assets in the system.
Mailbox providers do not judge inboxes in isolation. Domain history, mailbox behavior, IP and tenant reputation, content, volume patterns, complaints, and bounces can all matter. Put fifty senders on one domain and each sender shares more exposure to problems created by the others. One bad list or aggressive sequence can affect more capacity at once.
Low density is not a guarantee. It is a deliberate way to reduce how much capacity depends on any one domain.
Mailrun caps density at ten inboxes per domain as a conservative operating guardrail. That spreads the same volume across more domains, with several practical benefits:
It costs a little more in setup and management. That's exactly the work Mailrun automates, so the safer architecture isn't also the slower one.
High density can optimize the purchase price while concentrating operational risk. Mailrun chooses a ten-inbox ceiling, keeps density lower when practical, and scales sideways so one weakening domain represents a smaller portion of the program.
Size domains and density to your sending target and see the capacity that holds.
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