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Mark, I am hopeful you found something interesting. Yes, I have tried to use just port 25 and no authentication. Internal and external email addresses produce the same error. Yes, the problem happens regardless of address. ports, security, server settings, account info. Well, similar, Outlook 2010 changed their settings a bit but the gist is the same. Yes, Outlook 20 have all the same settings. I am back to Outlook on Windows 10 being the problem.

So, I think we can take the server out of the equation.

'Error 551 - this server requires authentication before sending email from a locally hosted domain.' Again, Outlook from Windows 7 works. Yes, it was under a different domain, but everything else is the same. Just for grins I setup this users account on this server. This is a 2012 R2 Server with the latest version of Mail Enable. I happen to have another server with Mail Enable. I too think that Jim is onto something with TLS. The mail server logs just say invalid user name or password. Works perfectly.ĭoes Windows 10 use some sort of different encryption behind the scenes for a handshake with the mail server. The kicker is that on these machines where Outlook is failing and cannot send email I can install Thunderbird and it goes through without a single hiccup. I have started with brand new outlook pst files and I have tried it by restoring the pst file from the old Windows XP machine we upgraded from. And I have made sure to verify all the settings. I have Outlook working on Windows 7 pro 64 bit with both the Office 2007 and Office 2010 32 bit configurations. I keep getting the '551 this mail server requires authentication' error. But I cannot for the life of me get outlook to send email. I have setup a great number of email accounts over the years so I don't consider myself a rookie. I have tried both Office 2007 and Office 2010 (both the 32 bit version) I recently setup some Windows 10 Pro 64 bit systems in our office and I am trying to get Outlook to work.

Thunderbird 64 bits windows 10