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What it does

The Send Email tool lets your agents send HTML-formatted emails to one or multiple recipients, optionally attaching files from the current conversation. Suitable for notifications, daily/weekly reports, alerts, and any automated outbound communication. By default, emails are sent through Aster’s shared sender ({Agent Name} <agent-{id}@updates.asteragents.com> or your agent’s email slug if configured). Connect Resend to send from your own verified domain instead — the tool picks up the connection automatically with no agent changes.

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Key features

  • Send to one or multiple recipients in a single call
  • Full HTML body support with inline styling, lists, tables, links
  • Attach files from the conversation by filename — generated reports, CSVs, PDFs, anything produced by other tools
  • Agent name is used as the display name automatically — different agents in the same org show as different senders
  • Works with Resend BYO — when your org has a Resend connection, the tool uses your API key and verified domain transparently

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Parameters

ParameterTypeRequiredDescription
toarray of stringsYesRecipient email addresses
subjectstringYesEmail subject line
htmlstringYesHTML body (raw HTML, no XML wrapper)
attachment_filenamesarray of stringsNoFilenames of conversation files to attach (e.g. ["report.xlsx", "data.csv"]). Each name must match a file already present in the conversation — uploaded by the user or produced by another tool. Files not found in the conversation are skipped with a warning.

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From-address behavior

The from-address an agent sends as depends on whether your org has a Resend connection:
Org connection stateFrom-address
No Resend connection (default){Agent Name} <{agent-slug or agent-{id}}@updates.asteragents.com>
Resend connected (BYO){Agent Name} <{your configured from-email}>
In both cases the agent’s name is used as the display name. With BYO Resend, all agents in your org share the same configured mailbox but are differentiated by display name in recipients’ inboxes.

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Common use cases

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Send a notification

to: ["user@example.com"]
subject: "Task Completed Successfully"
html: "<h2>Task Update</h2><p>Your data analysis task has been completed successfully. Please check the results in your dashboard.</p>"

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Send a report with an attached file

to: ["manager@company.com", "team@company.com"]
subject: "Weekly Sales Report"
html: "<h1>Weekly Sales Report</h1><p>Summary attached. Top-line numbers in the body.</p><ul><li>Total: $15,000</li><li>Top product: Product A ($8,000)</li></ul>"
attachment_filenames: ["weekly_sales_2026-06.xlsx"]
The agent typically produces the file via execute_python or another tool earlier in the conversation, then references it by exact filename here.

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Send an alert

to: ["admin@company.com"]
subject: "System Alert: High CPU Usage"
html: "<div style='color: red;'><h2>System Alert</h2><p>CPU usage has exceeded 90% for the past 10 minutes.</p></div>"

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Send a daily digest from a scheduled task

Daily/weekly agents commonly use send_email as the final step of a scheduled run — the agent gathers data, formats a digest, and emails it to a distribution list. Scheduled tasks pick up your Resend connection automatically; no extra config beyond connecting Resend once.

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HTML formatting tips

  • Use standard inline HTML — <h1>, <h2>, <p>, <strong>, <em>, <a href="…">, <ul>, <ol>, <table>
  • Prefer inline style="…" over <style> blocks — many email clients strip the latter
  • Keep CSS simple; avoid flexbox/grid for layout (poor email-client support)
  • Test on Gmail and Outlook for any high-volume sending

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Best practices

  • Keep subject lines short and specific — recipients see them before the body
  • For high-volume sending or external recipients, connect BYO Resend so your domain’s reputation is what carries deliverability
  • For attachments, generate the file with execute_python or another tool earlier in the conversation, then reference it by exact filename
  • Resend’s per-message attachment cap is 40 MB total across all attachments — oversize files are skipped with a warning rather than rejecting the whole send

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Troubleshooting

“No recipients provided in ‘to’”
  • The to parameter must be a non-empty array of strings (e.g. ["a@b.com"])
“Missing or empty ‘subject’ / ‘html’”
  • Both subject and html are required and must be non-empty strings
“Resend rejected the test send” (during BYO setup) Email delivers but content looks broken
  • Email clients aggressively strip CSS — stick to inline styles and standard tags
  • Use a litmus test recipient to preview before broad sends
Attachment not included in the sent email
  • The attachment_filenames entry must match exactly the filename of a file already in the conversation (case-sensitive, including extension)
  • Files generated by tools earlier in the conversation are referenceable by their filename — check the tool result for the exact filename it produced
  • If the file is missing from the conversation, the tool logs a warning and proceeds without the attachment rather than failing the send
Agent sending from updates.asteragents.com when I expected my own domain
  • Confirm Resend is connected under Control Hub > Integrations > Resend — the card should show Connected
  • See the Resend integration troubleshooting for connection issues
  • Resend Integration — connect your own Resend account so agents send from your verified domain
  • Schedule Task — scheduled agents typically end with send_email to deliver a digest
  • Execute Python — generate report files in the conversation, then attach them via attachment_filenames
  • Email Your Agent — let users email an agent and get an emailed reply back (uses the same outbound path, including BYO Resend)