How to Electronically Sign an Email | S/MIME vs. Signed Attachments

Electronically signing an email means either cryptographically signing the email message itself with an S/MIME certificate or signing an attached document like a PDF using a platform such as DocuSign or Signeasy and then sending the signed file.

What Does It Actually Mean to Electronically Sign an Email?

Electronically signing an email covers two different actions, and confusing them is the most common misstep. One method cryptographically signs the message text itself, proving that the sender is who they claim to be and the message content hasn’t been tampered with. The other method simply signs a document file (like a contract or form) attached to the email and then sends the signed version back. Each one serves a different legal and practical purpose.

Signing the Email Message Itself with S/MIME

This method, known as S/MIME (Secure/Multipurpose Internet Mail Extensions), uses a digital certificate to create a cryptographic signature on the email itself. Recipients can verify both the sender’s identity and the integrity of the message content. It is the only method that technically “digitally signs” the message text.

What You Need for S/MIME

You need an S/MIME digital certificate, also called a Digital ID, issued by a trusted certificate authority like DigiCert, Sectigo, or Identrust. These typically cost between $50 and $150 per year for an individual subscription. The certificate is usually downloaded as a .p12 or .pfx file that requires a passphrase. The recipient must also have a valid certificate for their email client to verify your signature automatically.

How to Set Up S/MIME in Gmail

1. Purchase and install an S/MIME certificate from a provider like DigiCert. Import the .p12 file into your computer’s certificate store — on Windows, use the Certificate Import Wizard; on a Mac, import it into the Keychain.
2. Open Gmail Settings > click the Gear icon > See all settings > General.
3. Scroll to S/MIME and check “Add a digital signature to all messages I send.”
4. Send a test email to yourself. A red wax seal icon on the message header confirms the digital signature is active and working.

How to Set Up S/MIME in Outlook (Desktop)

1. Import your certificate into the Windows certificate store first if you haven’t already.
2. Open Outlook > File > Options > Trust Center > Trust Center Settings > Email Security.
3. Under Digital IDs and Certificates, the imported certificate should appear. Check “Add a digital signature to all messages I send.”
4. Compose and send a test message. A ribbon or shield icon on the sent message indicates the signature is present.

How to Set Up S/MIME in Apple Mail

1. Install the certificate into your Mac’s Keychain first. Double-click the .p12 file and follow the prompts to add it.
2. Open Mail > Mail menu > Preferences > Signatures.
3. Select your email account and click the “+” button to create a new signature. Mail automatically associates the certificate with the account once installed.
4. Send a test message. A signed icon appears on the message in the sent folder.

Signing a Document Attached to an Email

This is the practical route most people need. Instead of signing the email text, you sign a document (typically a PDF or Word file) that is attached to the email, then attach the signed version to your reply. Signeasy’s Gmail add-on is one of the most straightforward tools for this, and works right inside your inbox.

Using Signeasy Inside Gmail

1. Install Signeasy from the Gmail Workspace Marketplace. Grant the permissions it requests to access your email attachments.
2. Open the email that has the document attached.
3. Click “Sign” on the attachment preview, then select “Sign Yourself.”
4. Choose how to sign: Draw your signature with your mouse or trackpad, Type it out in a handwriting-style font, or Upload an image of your physical signature.
5. Place the signature on the document’s signature line, then save and send the signed document as a reply to the original email thread.

Using Adobe Fill & Sign (Desktop or Mobile)

1. Open the attached PDF in Adobe Acrobat Reader. The free version includes the Fill & Sign tool.
2. Click the “Fill & Sign” tool in the right pane.
3. Select the signature icon at the top. You can draw, type, or upload a stored signature image.
4. Click the signature line in the document to place it, then drag to the correct position if needed.
5. Save the signed PDF locally, then attach it to a reply email and send.

Using DocuSign

1. Create a free DocuSign account and download the mobile app or use the web interface.
2. Upload the document from your email or device.
3. Select “Sign Yourself” and place your signature fields on the document using the drag-and-drop interface.
4. Finish and download the completed document. Attach it to your email reply.

Quick Comparison: Which Route Should You Use?

The table below helps you decide between the two broad approaches based on what you actually need.

Common Mistakes That Break an Electronic Signature

Even when using the right tool, a few missteps can make your signature invalid or unverifiable. The most frequent ones involve the certificate setup itself. Many users install the certificate but forget to enable the signing toggle in their email client, so the signature never gets applied. Others lose the .p12 file or the passphrase, and without those, the certificate cannot be reinstalled on a new machine. On the document-signing side, using an e-signature platform that does not produce an audit trail can leave you without proof of intent, which matters under the US ESIGN Act. Always save a copy of the signed document with its audit trail if the platform provides one.

Final Decision Checklist

• Do I need to sign the email message itself? Yes → Use S/MIME with a purchased certificate and compatible email client. Both you and the recipient need valid certificates.
• Do I need to sign a document sent as an attachment? Yes → Use Signeasy (Gmail), Adobe Fill & Sign, or DocuSign. Download the signed file and attach it to your reply.
• Am I signing on mobile? Use Adobe Fill & Sign or DocuSign’s mobile app for drawing with your finger.
• Is this a legal contract that might be challenged? Use DocuSign or Adobe Acrobat Sign. Both produce detailed audit logs that comply with the US ESIGN Act.