SendAs RelayHow SendAs Relay works
SendAs Relay is a browser extension plus a small relay. The extension adds a From menu to Gmail on the web. When you pick one of your other addresses, it builds the email itself and has your provider's own SMTP server send it, the same server a mail app like Thunderbird or Apple Mail would use. Gmail never sends it.
What happens when you click Send
- 1. Gmail composeYou pick your address in the From menu and click Send or press Ctrl+Enter.
- 2. ExtensionStops Gmail's own send, reads the recipients, subject, text, images and attachments, and builds a standard email.
- 3. RelayA Cloudflare Worker opens an encrypted connection to your provider and passes the message on. It keeps nothing.
- 4. Your providerYahoo, iCloud, Fastmail or your domain's SMTP server signs you in and delivers the message.
- 5. Gmail SentOptional: the relay saves a copy into Gmail's Sent Mail over IMAP, so your sent mail stays in one place.
Then the extension discards Gmail's draft and shows "Sent from you@yourdomain.com".
Why a relay is needed
Browsers can't open SMTP connections; only web requests are allowed. So the extension sends each message over HTTPS to a relay, which speaks SMTP to your provider. The relay is stateless: it reads the request, holds the connection open while your provider accepts the message, returns the result and forgets it. There is no database, no message queue and no logging.
You can use the hosted relay, or deploy your own copy on a free Cloudflare account and paste its URL into the settings. The relay code is open source (MIT): published on GitHub at launch (link will appear here).
What the extension reads in Gmail
- Nothing, until you click Send with another address selected. With your Gmail address selected, Gmail sends as usual and the extension stays out of the way.
- On Send: the To, Cc and Bcc fields, the subject, the message body, pasted images and the attachments in that compose window.
- For a reply: the original message's headers (Message-ID and References), fetched from your own Gmail session through Gmail's "Show original", so the reply stays in the same conversation for you and the recipient. If they can't be read, the reply still goes out and you get a note that it may start a new conversation.
What recipients see
Your name and address, exactly as when you send from your provider's own webmail. The message leaves from your provider's servers, so it's signed and checked as your provider's mail (SPF, DKIM), with no "via gmail.com" label. Bcc recipients get the message, but the Bcc list isn't in the copy anyone receives.
Copies in Gmail Sent
Gmail doesn't see mail that another server sends, so the extension offers three options:
| Option | What happens | What you need |
|---|---|---|
| Save to Gmail Sent (recommended) | After sending, the relay uploads a copy into [Gmail]/Sent Mail with IMAP. It keeps the Bcc line so you can see who got it |
A Gmail app password |
| Bcc to my Gmail | Your Gmail address is added as a hidden recipient. The copy arrives in your inbox; a filter can label it | Nothing extra |
| No copy | Only your provider's Sent folder has it, if it keeps one | Nothing |
More detail: how the Sent copy works.
Limits
| Attachments | Up to 14 MB in total per message. Files added with Gmail's paperclip are included when the extension can read them; if not, it refuses to send and asks you to use its own Attach button, rather than sending without them |
| Message size | 20 MB encoded |
| Recipients | 100 per message |
| Sending rate | 20 messages a minute per address (hosted relay), on top of your provider's own daily limits |
| Ports | 465 (TLS), 587 and 2525 (STARTTLS). Port 25 is never used |
| Browsers | Firefox 140+ and Microsoft Edge 137+ on desktop |
What it can't do
- Outlook.com, Hotmail and Live addresses: Microsoft allows only Microsoft sign-in (OAuth) for them since 16 September 2024. Support is planned. Options for now.
- Proton Mail: SMTP is available only through Proton Bridge on your own computer.
- Scheduled send, undo send and confidential mode for your other addresses. Your Gmail address keeps all of them.
- The Gmail apps for Android and iOS. It's a desktop browser extension.
- Incoming mail. Set up forwarding from your other provider to Gmail; see the POP3 alternative guide.
When Gmail changes its layout
The extension looks for Gmail's compose window by its structure. If Gmail changes in a way the extension doesn't understand, it refuses to send and says so ("can't read this compose window"), and you can send with Gmail or update the extension. It never sends a message it couldn't read completely.