Sending mail

mail_send, mail_reply, mail_forward and mail_send_draft send from one account. Use mail_validate_send to learn whether a message would be sent, held for approval or refused, and why, without sending it.

Limits and rules

Each account has a daily send cap: 100 by default, and 20 in a new workspace's first 24 hours, when each message or invitation also reaches at most 10 people; it resets at 00:00 UTC. The account's policy can block recipients, allow only some, or hold sends for approval.

Attachments

Attach small files inline (base64, at most 3 MB in all), and larger ones with an upload token: attachments_generate_upload_link gives the user a page to drop files on (valid 30 minutes, up to 150 MB in all), and attachments_check_upload_status returns the upload tokens. Uploaded files are deleted after 24 hours. A message's files together may be as large as the account's own provider allows:

Account	Attachments per message
Microsoft 365	Up to the mailbox’s own limit: 35 MB by default, which a Microsoft 365 admin can raise to 150 MB.
Outlook.com	Outlook.com takes attachments up to 25 MB.
Gmail	Gmail takes up to 25 MB of attachments per message.
Zoho Mail	Zoho Mail takes 20 MB per email on free plans and up to 40 MB on paid ones.
Fastmail	Fastmail takes emails up to 70 MB, about 50 MB of attachments once encoded.
iCloud Mail	iCloud Mail takes messages up to 20 MB.
Yahoo Mail	Yahoo Mail takes up to 25 MB of attachments per message.
GMX	GMX takes attachments up to 50 MB.
AOL, and other mail servers	Up to 25 MB, or less if the mail server announces a smaller size.
Demo mailbox	The Demo mailbox takes up to 3 MB of attachments per message.

Checked against each provider's documentation on 2026-10-25. Where an account is set lower (a Microsoft 365 admin's limit, a smaller Zoho plan, a mail server's announced size), the provider's own refusal comes back as ATTACHMENT_TOO_LARGE.

Retries

Pass idempotency_key on sends: the same key within 24 hours returns the first answer instead of sending again.