Sendmux is an email layer that routes cold outreach through inboxes you own. Add sending accounts, set per-account caps, and run your GTM motion from an AI tool.
Sendmux is an email layer that routes cold outreach through sending accounts you already own. You add each inbox, set its sender details and its per-hour and per-day caps, then create one sending key. Sendmux picks an eligible account for every message and holds anything over that account’s current cap.That makes it the send rail underneath a GTM motion. Your tools pick the targets and write the copy. Sendmux decides which inbox carries each message and how fast.
You need a Sendmux team, at least one inbox you can supply SMTP credentials
for or a Gmail or Outlook account you can authorise, and a prospect list you
may lawfully contact.
Managed Amazon SES is for transactional email only. Use your own provider for
marketing, newsletters, or bulk promotional sends.
No. You do not add or verify a domain in Sendmux when you send through your own accounts. Each account already authenticates for its own domain at its own provider, so Sendmux hands the message to that provider and the provider signs it.
What you want to do
Domain needed in Sendmux
Send through your own accounts
No
Send through the managed SES from Sendmux
Yes
Receive mail or host mailboxes in Sendmux
Yes
One failure mode is worth knowing. If the managed account is the only eligible
provider for a send and the sender domain is not verified, Sendmux rejects the
message and records it in Logs rather
than returning a bounce.
Open Accounts and select Add Account, then choose Custom SMTP,
Gmail, or Outlook / Microsoft 365. Run Test Connection on SMTP
accounts before you route real traffic through them.Any inbox provider that gives you SMTP credentials works as a Custom SMTP
account, including Mission Inbox, ScaledMail, Zapmail, and others. There is
no vendor-specific connector for these. You supply the host, port, username,
and password, and Sendmux treats the account like any other.
Choose the connection type when you add a sending account.
Running a pool of inboxes? Use Import Accounts with the CSV template
instead of adding them one at a time. See
Import accounts from a file.
A pool of inboxes, each with its own delivery group and per-minute, per-hour and per-day caps.
Then set the per-account details that make a pool behave like separate
senders:
Setting
What to set it to
From email and From name
The persona that owns this inbox.
Reply-To
Where replies should land.
Quotas
Per-second, minute, hour, or day caps. 0 means unlimited, and a range such as 5-10 is accepted.
Distribution percentage
0 to 100, default 100. Weights traffic when several accounts are eligible.
Advanced settings hold the sender identity, the caps and the routing weight for one account.
Your sending route decides which credential you need, so choose before you
create anything.
Route
Credential
SMTP, from an outreach tool or script
A sending key
HTTP, from your own code
A sending key
An AI client over hosted MCP
None. You authorise a connected app instead
Sending through more than one route is fine. Create a key for the routes
that need one and authorise MCP separately.
3
Create a sending key, for SMTP and HTTP
Skip this step if you only plan to send over hosted MCP.Open API Keys, select Create API Key, and choose the Sending key
type. Pick whether the key routes through all providers, selected providers,
or a delivery group.
Choose Sending Key. It covers the API, the CLI, MCP and AI agents.
Copy the smx_mbx_ value straight into your secrets manager. It is shown
once. See Create a key and
Key types.Then send:
SMTP. Host smtp.sendmux.ai, port 587 with STARTTLS, password is
your sending key. See Sending by SMTP.
Hosted MCP authorises a connected app and issues its own short-lived tokens.
It does not use a sending key, and connected apps never create smx_mbx_
secrets. See Connected apps and the next
section.
Contact only people you have a lawful basis to email. Say who you are, keep
the message relevant to the recipient, honour every suppression record, and
give people a clear way to stop hearing from you.
Connect Sendmux to Claude or another MCP client and your agent can send through the same accounts, with the same caps, without you pasting a key into it.
1
Add the server
Add https://mcp.sendmux.ai/mcp in your client. Per-client instructions are
in Hosted OAuth clients.
2
Authorise the connection
Sendmux asks you to pick a team, then which product lines the app may use.
Grant Sending API only. Every product line is ticked when the
authorisation screen opens. Untick Mailbox API and Management API so
Sending API is the only one left. A Sending-only grant never asks you to
choose mailboxes, and you do not need to create one. Add Mailbox API later
if you also want the agent to read replies.
Under Product lines, leave Sending API ticked and clear the other two.
For what each surface covers and how long tokens last, see MCP.
Use a fixed cap when you want one predictable ceiling per account. Use a range when you would rather the ceiling move inside boundaries you set. Sendmux picks a value when account information refreshes and holds that cap until the next refresh.A range varies the ceiling. It does not add a delay between individual messages, and it does not guarantee inbox placement. Targeting, authenticated sender domains, message quality, and account history all still decide what happens after Sendmux hands the message over.
Distribution percentage weights traffic between accounts that are eligible for the same send. Delivery groups go further and keep a stream of traffic on a chosen subset of accounts. The key you authenticate with decides which applies. See How routing works.
Put each campaign, or each customer, on its own delivery group. One inbox
going bad then affects that group and nothing else.
Replies land wherever the Reply-To on each account points, which by default is the inbox provider you own. Nothing in Sendmux is required for that.You only need a Sendmux domain and mailbox when you want replies inside Sendmux, so an agent can read and act on them. That is also when the Mailbox API grant becomes worth adding to your MCP connection. See Domain management and Mailboxes.
No, not when you send through your own accounts. Each account authenticates
at its own provider. A verified domain in Sendmux is needed only for the
managed account, or to receive mail.
Which inbox providers work with Sendmux?
Any provider that gives you SMTP credentials, plus Gmail and Outlook or
Microsoft 365 through the connected-account flow. Cold email inbox providers
such as Mission Inbox, ScaledMail, and Zapmail connect as Custom SMTP
accounts.
Do I need a mailbox to use MCP for sending?
No. A Sending-only grant does not ask you to choose a mailbox, and you do not
need to create one. Add the Mailbox API product line only when you
want an agent to read replies.
How many sending accounts can I add?
Each team starts with 200 accounts you add yourself. The managed account does
not count towards that. See
Account limits and
Default limits.
Can I use the free managed account for cold outreach?
No. Managed Amazon SES is for transactional email only. Cold outreach,
marketing, and bulk promotional sending must go through accounts you own.
How do I see what was delivered?
Open Logs. You can filter by status, account, and recipient, and export
the result. See Filter logs.
