Microsoft 365

You add a Microsoft 365 mailbox to Mailway as Any SMTP server. Mailway signs in to Exchange Online with the mailbox’s credentials and sends from it (Microsoft calls this client SMTP submission).

Sign-in changes in December 2026

Mailway signs in with a username and password. Microsoft keeps this working until the end of December 2026. After that, it is off by default for existing tenants (an admin can turn it back on) and not available for new tenants. Microsoft will announce the date it removes it completely in the second half of 2027. Mailway doesn’t support sign-in with OAuth, the method Microsoft recommends, yet. Plan for this before you rely on a mailbox.

Before you start

• A licensed mailbox to send from, with Authenticated SMTP (SMTP AUTH) turned on for that mailbox. It is off for organizations created after January 2020. An admin turns it on per mailbox in the Microsoft 365 admin center (Users → Active users → the user → Mail → Manage email apps).
• Microsoft Entra security defaults block SMTP sign-in with a password. If your tenant uses them, this method won’t work.
• To send from an address other than the mailbox’s own, the mailbox needs Send As permission for it. Otherwise Exchange returns 5.7.60 Client doesn't have permissions to send as this sender.

Add the account in Mailway

1. Add the provider.

   In the console, open Providers → New provider and pick Any SMTP server. Fill in:

   Field	Value
   Server	smtp.office365.com
   Port	587
   Encryption	STARTTLS
   Username	the mailbox’s sign-in address
   Password	the mailbox’s password
   TLS verification	leave it on

   Secrets are encrypted at rest.

2. Attach it to a project.

   Open the project and attach the provider. To use the mailbox only as a backup, attach it last and set the project’s routing strategy to fallback (see Limits).

Delivery events

Mailway doesn’t read delivery reports from Microsoft 365, so the last state a mail can reach is accepted, when Exchange Online takes it. Bounces and complaints that Microsoft learns about later don’t reach Mailway, and they aren’t added to the project’s suppression list.

Check the connection

The provider page’s connection check signs in to Exchange Online’s SMTP server and closes the connection without sending anything. If SMTP AUTH is off for the mailbox, or security defaults are on, the check fails at sign-in.

What Microsoft 365 changes

Exchange Online saves a copy of every mail in the mailbox’s Sent Items folder. Mailway’s own archive always keeps the message as your app sent it.

Note

Microsoft 365 hasn’t been through Mailway’s provider tests yet, so this page lists what Microsoft documents rather than what we measured. The tested providers are in What providers change.

Limits

A mailbox is not a bulk sender. Exchange Online allows 30 messages a minute and 10,000 recipients a day per mailbox, and refuses mail over those limits. When it refuses a mail, Mailway fails over to the next provider attached to the project. With the default random strategy the mailbox gets a share of all the project’s mail, so a busy app reaches the limits quickly. With fallback and the mailbox attached last, it only gets mail that the providers before it refused.