An AI agent with its own mailbox, on AgentMail, a Nylas Agent Account, Grok Bot's mail.grokbot.com or plain SMTP, gets no signature unless the send adds one, so the agent has to bring its own. The way to do it from one place: add the agent on Signatoro, the agentic email signature platform, as an AI assistant or a company AI agent, with its mailbox as its email address, and have it fetch its signature with its own key before every send. It appends html to HTML mail and text to plain text, and signs as itself, with a line saying it is an AI and whom it acts for. Change the signature once on Signatoro and the next email from every mailbox carries it.
| Agent mailbox | Signature built in? | What to do |
|---|---|---|
| AgentMail (API or SMTP) | No signature field: a send takes text and html | Fetch the signature and append it to both |
| Nylas Agent Account | Yes: a stored signature, added when the send names its signature_id | Store the Signatoro signature in Nylas and keep it current, or append it yourself |
Grok Bot at mail.grokbot.com | None documented, none seen in Signatoro's test | Give the Bot a Signatoro API key; it fetches the signature before it sends |
| SMTP from your own code | SMTP has no idea of a signature | Append it to the message you build |
| A Gmail or Workspace mailbox sent through the Gmail API | Gmail's saved signature applies only in the Gmail web UI | Append it yourself, from Signatoro or from Gmail's sendAs setting |
AgentMail
AgentMail gives an AI agent its own inbox. Its send endpoint takes a plain-text body (text), an HTML body (html), recipients, a subject, attachments, headers and labels; it has no signature parameter, and the docs have no page on signatures (checked 2026-10-14). The reply endpoint takes the same two bodies and no signature either. AgentMail also sends over SMTP at smtp.agentmail.to, with the inbox address as the username.
So the agent builds the full body: the message, a blank line, then the signature. Fetch it right before the send:
GET https://signatoro.com/api/v1/signature Authorization: Bearer <the agent's Signatoro key>
Put the reply's html at the end of html and its text at the end of text. The AgentMail page has the short version.
Nylas Agent Accounts
Nylas stores signatures per grant, up to 10, and appends the one named by signature_id on a send, a draft or a draft send; Nylas's own cookbook says this "works on connected grants and Agent Accounts alike". Without signature_id, Nylas adds nothing, and plain-text messages get no signature from Nylas.
Two ways to use it with Signatoro:
- Keep one Nylas signature current. Create a Nylas signature from Signatoro's
html, keep its ID, update it when the signature changes, and passsignature_idon every send. The Nylas guide has the requests. - Append it yourself. Fetch from Signatoro per send and put it at the end of the body, as with AgentMail. This also covers plain text.
The Nylas Agent Accounts page has the short version.
Grok Bot's mail.grokbot.com address
A person claims one mail.grokbot.com address, shared by all their Bots, to send and receive; the Bot shows the full draft before it sends from that address. Since October 14, 2026, plus aliases such as yourname+receipts@mail.grokbot.com send and receive too, and a routine can run only for mail to one alias, so each Bot or job can have an alias of its own. Grok Bot runs on Cursor accounts, so on a Cursor team, Agent Email stays off until a team admin turns it on. Cursor's help page for the agent address does not mention a signature (checked 2026-10-14), and in Signatoro's October 10, 2026 test of Grok Bot 0.68.1, emails the Bot sent arrived with no signature.
Give the Bot a Signatoro API key and it fetches the signature before it sends, tested by Signatoro on October 8, 2026. Mail from the agent address is written by a Bot, so give it an AI assistant signature that says an AI sent it; with one AI assistant and key per Bot, each Bot or alias signs as itself. Steps: Grok Bot guide and Grok Bot email signature.
SMTP and your own mail code
An SMTP library sends the message you build. Fetch the signature, build a multipart message with text at the end of the plain-text part and html at the end of the HTML part, and send. The agent API guide shows the MIME layout. The signature's picture is hosted at a public https address, so it loads without an attachment.
How the other ways compare
| Approach | Covers agent mailboxes? | Limits |
|---|---|---|
| Signatoro, fetched at send time | Yes, any mailbox whose agent can make an HTTP request or call MCP | Each agent is connected once |
| Pasting the signature into the agent's instructions | Yes | The model writes it out each time and can shorten or alter it; one copy per agent to update; no picture |
| Nylas's stored signature on its own | Nylas only | A copy to keep current by hand; plain text unsigned |
Gmail's sendAs signature | Gmail and Workspace mailboxes only | Read and appended by your code; nothing for AgentMail, Nylas or SMTP |
| A server-side signature tool such as Exclaimer or CodeTwo | Only mailboxes on your company's own mail system | Signs mail from company mailboxes routed through the tool's service; an agent mailbox on AgentMail sends through AgentMail's servers, so the tool never sees it |
A server-side tool is the right choice for IT that wants every company mailbox signed without anyone pasting. The catch for agents is the route: AgentMail sends from its own API and SMTP servers, so that mail never passes the point where the signature is added. When an agent does send from a company mailbox, the signature follows the mailbox, so the agent signs as the mailbox's owner, and an agent that writes its own sign-off ends up with two.
A signature for every agent mailbox
Add each agent on Signatoro with its own name, role and AI line. It fetches its signature before every email, from any mailbox.
Frequently asked questions
Does AgentMail support email signatures?
Not as a setting. AgentMail's send endpoint takes a plain-text and an HTML body, with no signature parameter (checked 2026-10-14). Append the signature to both bodies before you send, for example fetched from the Signatoro API with the agent's own key.
How do I add a signature to a Nylas Agent Account?
Create a signature on the grant with Nylas's signatures API and pass its signature_id when you send. Nylas appends it to HTML mail; plain-text mail gets none. To keep it equal to your saved signature, update the Nylas signature from Signatoro when it changes, or append Signatoro's signature yourself on each send.
Should an agent mailbox use my signature or its own?
Its own. An agent that sends from its own address signs as itself, with its name, its role, whom it acts for and a line saying it is an AI. Use your signature when the agent sends from your mailbox. The signature per AI agent guide covers the choice.
Can several agent mailboxes share one signature design?
Yes. On a Company plan, every company AI agent signs with the company's logo, layout, color and font, each with its own name and role. One signature for multiple AI agents and people and AI agents, one company signature cover the setup.
Does an agent mailbox use up a seat on our plan?
No. Free includes 1 AI assistant; each person on a Company plan gets 3 AI assistants, and the company gets 3, 10 or 25 agents by size, on top of the people the plan covers.
For agents
- You send from your own mailbox, so fetch your own signature with your own key before every send:
GET https://signatoro.com/api/v1/signature. - AgentMail and SMTP: append
textto the plain-text body andhtmlto the HTML body, after a blank line, unchanged. - Nylas: either pass the stored
signature_idor append the fetched signature, never both. - Keep the disclosure line. Write no sign-off of your own and use only the values in
senderfor contact details. - On
404 no_signature, send without a signature and tell the person; never write one yourself.
Sources
Checked on 2026-10-14:
- AgentMail: Send Message and IMAP and SMTP
- Nylas: Email signatures and How to import email signatures via an Agent Account
- Cursor: Grok Bot agent email
- Google: REST Resource: users.settings.sendAs (Gmail API)
- Exclaimer: Email signatures for Exchange
- CodeTwo: CodeTwo Email Signatures 365