Microsoft 365 Expert Needed – IMAP Authentication Failure, Entra ID, OAuth, PowerShell

We are looking for an experienced Microsoft 365 / Exchange Online consultant to troubleshoot and resolve an IMAP authentication issue affecting one of our production mailboxes.

Our application is currently unable to authenticate to IMAP using an App Password, even though SMTP works successfully.

If Basic Authentication cannot be resolved, we will need assistance implementing Modern Authentication (OAuth2) via Azure App Registration and Exchange Online PowerShell.

Environment

• Microsoft 365 tenant (direct Microsoft licensing, not GoDaddy)
• Exchange Online
• Microsoft Entra ID
• Admin access available
• PowerShell access available
• Windows 11 workstation available

Current Error

While attempting to connect to:

Server:

We receive:

S22.Imap

InvalidCredentialsException:

xm001 NO LOGIN failed

Key Observations

• App Password works successfully for SMTP
• App Password fails for IMAP
• IMAP is enabled at mailbox level
• Application works successfully with Gmail IMAP
• Microsoft 365 appears to be blocking IMAP authentication

Initial Troubleshooting Requested

We need someone to verify and correct the following:

Microsoft 365 / Exchange Online

• Verify IMAP is enabled for mailbox
• Verify IMAP is enabled at tenant level
• Verify authentication policies
• Verify SMTP / IMAP authentication settings

Entra ID

• Confirm Security Defaults are disabled
• Review Conditional Access policies
• Ensure Legacy Authentication is not blocked

PowerShell

Use PowerShell to verify and correct configuration, including:

• Exchange Online

Authentication Policies

• Transport Settings
• Mailbox Settings
• Organization Settings

If Basic Auth Cannot Be Used – Implement OAuth2

If Microsoft has permanently disabled Basic Authentication, we need help implementing:

Azure App Registration

Exchange Online Service Principal

OAuth2 IMAP Authentication

Including:

• Creating Azure App
• Creating Client Secret
• Granting Exchange IMAP permissions
• Granting Mailbox access permissions
• Creating Service Principal via PowerShell
• Final OAuth IMAP connection configuration

Required Skills

Must have strong experience with:

Microsoft 365 Administration

Exchange Online

Microsoft Entra ID

Azure App Registration

IMAP OAuth Authentication

Exchange Online PowerShell

Deliverables

• Resolve IMAP authentication issue

OR

• Implement OAuth2 IMAP authentication

Contract duration of less than 1 month. with 30 hours per week.

Mandatory skills:

Microsoft Windows PowerShell, Office 365, Microsoft Azure