Cold email for agencies that scales clients without burning domains

The short answer

An agency running cold email for multiple clients has one structural problem a single-company sender does not: reputation is per domain, so every client needs its own sending domains and mailboxes, and the cost of the operation scales with client count rather than with contact volume. The common mistake is sending for several clients from domains the agency owns, which feels efficient and is the fastest way to lose all of them at once, because one client with a weak offer drags the shared reputation down and every other campaign degrades with it. Client domains should be registered and authenticated under the client, both so reputation stays isolated and so the client keeps the asset when the engagement ends. Per-seat pricing also breaks down at agency scale, since an agency needs mailboxes far more than it needs seats: a five person team running twelve clients may need over a hundred mailboxes and five logins.