535: 5.7.3 Authentication unsuccessful

Same issue here, also in UK.

This was working fine for us until we enabled "Secure Defaults" in Azure Active Directory. We then came back and enabled the "Authenticated SMTP" setting against the mailbox, but it is currently still failing. I am hoping the settings update just takes a while to propagate but the docs do not make that clear.

I tried this. I was using smtp.office365.com and port 587. However I switched to [domain].mail.protection.outlook.com and port 25 as advised and now I see this error:

The SMTP server does not support authentication.

This seesm to have occurred after I enabled "secure defaults" in Azure AD. I have enabled SMTP Authentication for the domain user's mail box who I am attempting to authenticate with.

Do I need to also turn SMTP auth on at the organisation / tenant level?

Update: it looks like this person encountered the same issue and found a solution: 

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

I'll create a seperate answer.

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.

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

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.