535: 5.7.3 Authentication unsuccessful

After going back and forth with GoDaddy I found the solution in a mix of Microsoft Articles.  Here's the easiest way to enable the accounts:

**Open Powershell AS ADMINISTRATOR and enter the following commands:**Set-ExecutionPolicy RemoteSigned

winrm quickconfig

winrm get winrm/config/client/auth

At this point you should see something that says "Basic = true"

$UserCredential = Get-Credential

A popup box should appear, enter an administrators username and password for Exchange (e.g. emailaddress password)

$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection

Import-PSSession $Session -DisableNameChecking

get-CASMailbox -Identity USEREMAILADDRESS | fl SmtpClientAuthenticationDisabled

Set-CASMailbox -Identity USEREMAILADDRESS -SmtpClientAuthenticationDisabled $false

Remove-PSSession $Session

My original post was cut off so I re-edited.

Took me 4 attempts with GoDaddy and only resolved because I told them what to do following reading this forum. 11 days after Brian's issue they still haven't managed to communicate the issue to their front line email \ office365 help desk.

I had a similar issue. The problem is that Microsoft doesn't automatically turn on SMTP servers anymore.  Talk to your mail hosting company (in my case, it's GoDaddy) and they'll turn them on. Took 3 tech support guys at GoDaddy to finally figure this out.

Hi,

We finally found issue. The issue we were facing was due to old version of MailKit. We have upgraded the package to latest and it solved the Authentication issue. 

We where user 1.22 before and upgraded to 2.7.0 latest. 

Hope this help who ever using MailKit for sending Emails. 

Best Regards

Vishwas

Read this: 

https://community.spiceworks.com/topic/2276644-everything-using-office-365-smtp-authentication-is-broken-wont-authenticate

For reference: 

> We had the "Block Legacy Authentication" conditional access policy enabled for POP/IMAP/SMTP on all of our mailboxes which was overriding the per-mailbox SMTP Auth setting.  Added our backup mailbox as an exclusion in that policy, works now.  Nice.

> Instructions for those that might find themselves in the same boat...

> Log in to your Azure tenant and go to Azure Active Directory > Security > Conditional Access Policies.  This will show a list of all CAPs and their On/Off state.  Select one of the policies to view its settings.  Cllick Users and Groups to see who it applies to(probably all users).  Click Exclude and add mailboxes to exclude them from that policy.  In my case the policy was called Block Legacy Authentication, and I'm not sure if this is a default policy or something we added, but it was blocking pop/imap/smtp authentication for all lof our mailboxes and the fix was to exclude the mailboxes we want to use for SMTP auth, then in Office 365 admin portal > active users > select mailbox > mail > mail apps we enabled the SMTP Authentication setting for that mailbox.  That setting is overridden by the above CAP unless the mailbox is excluded from it.