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To send email with AgentPost, you need a mailbox on a verified custom domain. Default domain mailboxes (e.g., calm.brook-42@agent.agentpost.email) can only receive email — they cannot send.
If you have no custom domain mailboxes, you cannot compose outbound email. Add and verify a custom domain first, then create a mailbox on that domain.

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Prerequisites

  • A custom domain with status active or pending_warmup
  • At least one mailbox created on that domain

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Compose and send

1

Open the compose dialog

From any message view, click Compose. The compose dialog opens with a blank form.
2

Select a From address

Click the From dropdown. It lists all of your custom domain mailboxes whose domain can currently send (status active or pending_warmup). Select the address you want to send from.
3

Fill in the recipients

Enter one or more addresses in the To field. To add copied recipients, expand the CC / BCC section and fill in those fields.
4

Add a subject

Type your subject line in the Subject field.
5

Write the message body

Enter your message in the body field.
6

Attach files (optional)

Click the attachment button and select one or more files. Each file uploads immediately to storage and appears in the compose UI with its name and size. You can remove any attachment before sending by clicking the remove control next to it.
The per-attachment size limit is 25 MB. The total size of all attachments on a single message is also capped at 25 MB.
Uploaded files are held in a draft state (not yet linked to any message) until you send. If you close the dialog without sending, orphaned draft attachments are cleaned up automatically by a background job.
7

Send

Click Send. AgentPost creates an outbound Message record with send_status: pending and dispatches a background job to deliver the email. The status transitions to sent on successful delivery or failed if delivery is rejected.

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Attachment limits

LimitValue
Max size per file25 MB
Max total per message25 MB
Simultaneous uploadsMultiple (parallel)

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Sending with attachments via MCP

AI agents can also send emails with attachments using the SendEmail MCP tool. Instead of uploading files, agents pass base64-encoded file content directly in the attachments parameter. Up to 20 attachments are supported per message. See MCP Workflows — Attachments for a step-by-step example.

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What happens when you send

When you click Send, AgentPost queues your message for delivery. The email is dispatched through the outbound sending provider with the selected mailbox address as the From address — for example, support@example.com. You can monitor the send status in the mailbox’s sent items.