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Send from Gmail with your business address

Use Gmail to write from your Microsoft 365 address with the "Send mail as" feature.

Gmail's Send mail as feature on the web lets you use a Microsoft 365 sender through smtp365. smtp365 sends via Microsoft Graph; the message appears in Sent Items unless saving is disabled for the credential.

Google announces removal of this feature for third-party addresses from January 2027, with possible restrictions on new setups during 2026. This guide applies while the option remains available in your account. Use Outlook for your Microsoft 365 mailbox if Gmail no longer lets you add it.

Before you start

Connect your Microsoft 365 mailbox to smtp365, then create a dedicated credential in the dashboard and keep its username and password. Use these details, not your Microsoft 365 password.

Use the sender address configured for the credential: the connected mailbox or a shared mailbox verified with a test message. Any other address is rejected.

Keep the sender's mailbox open to receive the confirmation message. This setup is for sending only; it does not import incoming mail into Gmail.

Settings

  1. Open Gmail on a computer and, from the settings icon, select See all settings.
  2. Open Accounts and import, or Accounts in some accounts. Under Send mail as, choose to add another email address.
  3. Enter your name and the authorised sender. If the option to treat the address as an alias appears, leave it selected when the address is your own. Continue to the next step.
  4. Enter mail.smtp365.it as the SMTP server, 587 as the port and the smtp365 credential's username and password. Select the secure TLS connection, which uses STARTTLS. Alternatively, use SSL with port 465.
  5. Add the account and start address verification, if requested.
  6. Open the confirmation message in the Microsoft 365 mailbox or the authorised shared mailbox, for example in Outlook. Follow the link or enter the code in Gmail if offered.

Test

Write a new message, choose the authorised sender in From and send it to your own address. Check the sending log in smtp365 and Sent Items if saving is enabled. Also check the reply address in the added address's settings: Gmail may use your original Gmail address for replies.

If the device reports an error

Many devices show the code they got from the server. You will also find it in the dashboard's sending log, next to the message.

535

Invalid credentials

Wrong user or password, or the credential has been disabled. Copy the user and password again from the dashboard: if you no longer have the password, generate a new one. If you use port 25, check that it is enabled for that credential.

553

Sender not allowed

Each credential can only send with its own address. On the device, set the sender to the address of the connected mailbox, or of the shared mailbox you have verified.

454

Mailbox needs reconnecting

Microsoft asked for a new sign-in, for example after a password change. Open the dashboard and reconnect the mailbox: the device credentials stay the same.

451

Limit reached, try again later

Microsoft 365 or the credential's limit slowed sending down for a few minutes. Almost every device retries on its own; if it keeps happening, check the log to see who is sending so much.

552

Message too large

The message exceeds the mailbox limit, usually 35 MB. Lower the scan resolution or format, or split it into several sends.

550

Sending refused

The monthly quota is used up, the subscription is not active, or Microsoft 365 refused the sender. The text next to the code says which: if it is not clear, write to us.

The device cannot connect to the server at all? Check that the office firewall lets the chosen port out and that the device supports encrypted connections: some very old models need a firmware update.