One agent mailbox per tenant in a multi-tenant SaaS
Most multi-tenant SaaS apps that send email do it from one shared identity. There's a notifications@yourapp.com , every customer's mail flows through it, and the tenant is just a from_name you stamp on the subject line or a footer you swap out. That's fine until it isn't — until Tenant A's spam complaints drag down Tenant B's deliverability, until a reply from a customer lands in a single firehose inbox you now have to fan back out, until one tenant wants a stricter send cap than another and you realize you built none of that into the data model. So let's not share. Let's give every tenant its own real mailbox — a dedicated Agent Account per customer, each with its own grant_id , its own send identity, its own policy and limits, grouped into its own workspace. Not one inbox with a thousand label hacks. A thousand inboxes, isolated by construction. I work on the Nylas CLI, so the terminal commands below are the exact ones I reach for when I'm wiring this up. Every step gets the two-angle tour: the raw curl call and the nylas command that does the same thing. Why per-tenant beats one shared sender The shared-sender model fails along a few predictable seams. Per-tenant Agent Accounts close each one: Deliverability blast radius. When everyone sends from one address, one tenant's bounce rate and spam complaints poison the reputation everyone shares. Per-tenant accounts — and, if you want, per-tenant domains — keep one customer's bad behavior from sinking the rest. Inbound that actually belongs to someone. A shared sender means replies come back to one mailbox and you're left correlating them to tenants by hand. When each tenant has its own grant, an inbound message.created event already carries the grant_id . The routing is done before your handler runs. Per-tenant policy and limits. Different customers, different rules. A trial tenant capped at a low daily send; an enterprise tenant with a higher quota and longer retention. With a shared sender you'd build all of that y