Connect Microsoft 365

Connect a Microsoft 365 or Outlook mailbox via OAuth or an app password, including the SMTP AUTH admin requirement.

You can connect a Microsoft 365 mailbox two ways: OAuth (one click, when available) or an app password over SMTP/IMAP. The app password path has an extra wrinkle on Microsoft: SMTP AUTH must be enabled for the mailbox by an admin, and many tenants have it off by default.

Option 1: OAuth (one click)

When OAuth connect is available, choose Microsoft in the Add mailbox flow, sign in with the mailbox you want to warm, and approve access. No hosts, ports, or app passwords needed. Continue with the quickstart to configure warmup.

Option 2: App password over SMTP/IMAP

Prerequisites

Two things must be true before this path can work:

Two-factor authentication is enabled on the account. Microsoft only issues app passwords to accounts with 2FA (multi-factor authentication) turned on.
SMTP AUTH is enabled for the mailbox. An admin does this in the Microsoft 365 admin center: Users → Active users → select the user → Mail → Manage email apps → check Authenticated SMTP. It can also be enabled tenant-wide, but per-mailbox is the safer ask.

If SMTP AUTH is disabled, authentication fails no matter how correct your app password is — typically with an error mentioning "SmtpClientAuthentication is disabled for the Tenant". This is the single most common Microsoft 365 connection failure. Ask your admin to enable Authenticated SMTP for the mailbox first.

Create the app password

Sign in to your Microsoft account security settings.
Under additional security options, create a new app password (available once 2FA is on).
Copy the generated password — it's shown once.

Enter the settings

In the Add mailbox flow, choose SMTP/IMAP and enter:

Field	Value
SMTP host	`smtp.office365.com`
SMTP port	`587`
IMAP host	`outlook.office365.com`
IMAP port	`993`
Username	Your full email address
Password	The app password

Note the hosts differ between SMTP and IMAP — that's correct for Microsoft. Turn on use same credentials so the IMAP login reuses your username and app password, set your sender name, and run the connection test. It performs a real SMTP verification and IMAP login; nothing is saved until both pass.

Troubleshooting

"535 5.7.3 Authentication unsuccessful"

SMTP AUTH is disabled for the mailbox or tenant — the most likely cause. See the admin steps above.
You're using the account password instead of an app password.
Your tenant uses Security Defaults or a Conditional Access policy that blocks legacy authentication entirely. In that case app passwords won't work at all — use OAuth instead.