What you’re seeing
- Apps, scanners or website plugins fail to send email with '535 5.7.139 Authentication unsuccessful'.
- The error text may add that SMTP client authentication is disabled for the tenant or the mailbox.
- The same username and password still work to sign in to Outlook on the web.
- Multiple devices stopped sending at the same time, often after a Microsoft announcement or a security review.
Common causes
Every environment is different, but these are the causes our engineers find most often:
- Authenticated SMTP has been switched off for the entire Microsoft 365 organization, which is now the default for many tenants.
- The specific mailbox the device signs in with has SMTP authentication disabled at the mailbox level.
- Security defaults or a Conditional Access policy block legacy authentication for that account.
- Microsoft's retirement of basic authentication for client submission has reached your tenant.
- The account requires multi-factor authentication, which a basic username-and-password sign-in cannot complete.
Why it matters
Basic SMTP authentication is being phased out because stolen passwords make it easy to abuse. When it is cut off, every device and application still relying on it stops sending at once: contact forms on your website, CRM notifications, invoice emails and scanner deliveries. Customers stop receiving confirmations and staff stop receiving alerts. Turning the old method back on may buy time but leaves a known weak point in place, so the longer-term answer needs some planning.
How AskTechGurus fixes it
Skip the trial and error. A certified engineer works on your actual system, finds the root cause, and fixes it properly.
Tell us what’s happening
Submit a ticket in about two minutes. Paste the error, describe what changed, and pick your tier. No charge to submit.
A certified engineer diagnoses it
A vetted, US-based systems engineer claims your ticket, usually within minutes during business hours, and investigates over a secure remote session you can watch and end at any time.
Fixed and verified with you
The engineer resolves the root cause and confirms with you that it’s working. You don’t just get a workaround, and you get a plain-English summary of what was wrong.
Pay only when it’s resolved
Fixed price from $99, invoiced only after you approve the fix. If it isn’t resolved, you don’t pay.
Frequently asked questions
Is Microsoft forcing this change?
Microsoft has been steadily retiring password-only sign-in for email protocols for security reasons, and new tenants have it off by default. Exact timelines vary, so we check what applies to your tenant.
Which of our systems will be affected?
Anything that sends email using a mailbox username and password, such as scanners, websites and older business software. Part of the work is identifying all of them so nothing is missed.
Can this be sorted out remotely?
Yes. Tenant settings and most device configuration can be handled over a secure remote session, and we confirm each device is sending again before closing the ticket.