How to add your email signature to emails a Nylas agent sends

Nylas appends a stored signature to a message when the send request names its signature_id. This guide keeps that stored signature equal to the one the person saved on Signatoro, so mail from the agent carries the same company signature the team uses, with the company's footnote on plans that have one. Your agent does the work with the Nylas API key it already has, and that key stays with your agent.

If you need a signature first, make it free on Signatoro, then generate an API key on the signature page under "For tools and assistants", as the agent guide shows. An agent with its own mailbox is its own Signatoro person, with its own signature and key.

How Nylas adds a signature

Nylas documents its signatures API (checked October 2, 2026):

• A grant stores up to 10 signatures, each a name and an HTML body.
• Updating a signature keeps its ID, so one stored signature serves every send.
• Nylas appends the signature named by signature_id to the end of the message body on send, draft creation or draft send. Without signature_id, it appends nothing.
• Images must be external URLs, the body is capped at 100 KB, and Nylas strips tags and attributes it treats as unsafe. Signatoro's signature links its images by https URL.
• Plain-text messages get no signature from Nylas.

Store the signature once

Fetch the signature from Signatoro:

GET https://signatoro.com/api/v1/signature
Authorization: Bearer <SIGNATORO_API_KEY>

Create the Nylas signature from its html and keep the id Nylas returns, for example in the agent's config next to the grant ID:

POST https://api.us.nylas.com/v3/grants/<NYLAS_GRANT_ID>/signatures
Authorization: Bearer <NYLAS_API_KEY>
Content-Type: application/json

{ "name": "Signatoro", "body": "<html from Signatoro>" }

On every send

1. Fetch the signature from Signatoro, as above.

2. If its html differs from the body you stored last, update the Nylas signature in place:

   PUT https://api.us.nylas.com/v3/grants/<NYLAS_GRANT_ID>/signatures/<SIGNATURE_ID>
   Authorization: Bearer <NYLAS_API_KEY>
   Content-Type: application/json
   
   { "name": "Signatoro", "body": "<html from Signatoro>" }
   

3. Send with the signature_id:

   POST https://api.us.nylas.com/v3/grants/<NYLAS_GRANT_ID>/messages/send
   Authorization: Bearer <NYLAS_API_KEY>
   Content-Type: application/json
   
   {
     "subject": "Quarterly update",
     "to": [{ "email": "maya@northwind.co" }],
     "body": "<p>Hi Maya, ...</p>",
     "signature_id": "<SIGNATURE_ID>"
   }
   

A change the person or the company owner makes on Signatoro reaches the next email this way. Use the same host for every call: api.us.nylas.com or api.eu.nylas.com, wherever the Nylas application lives.

Replies and plain text

On a reply or forward, Nylas places the signature after the quoted text. To sign above the quote, leave out signature_id and put Signatoro's html into the body yourself, after your reply and before the quoted message.

For a plain-text message, append Signatoro's text to the body after a blank line, since Nylas adds no signature there.

Responses to branch on

If the Signatoro call fails, send without a signature or with the last one you stored, and tell the person. The agent guide lists each response: a revoked key, no saved signature, or a free account past its monthly requests.

Nylas signature questions

Does Nylas use the signature saved in Gmail or Outlook?

Nylas appends only a signature stored through its signatures API and named on the send. Its docs say a signature already set up in Gmail or Outlook is not synced, so store Signatoro's as above.

Can the Signatoro MCP server do this instead of the REST API?

Yes. An agent connected to https://signatoro.com/mcp calls get_signature and gets the same html and text, then stores and sends it the same way. The MCP guide covers connecting.