Hotmail.com is now part of Outlook.com - all merged to the same MX servers and networks as of January, 2015. Outlook is, apparently, using Hotmail.com, as the "experiment" to see how well their antispam measures are working as they work to tighten up their e-mail antispam and security.
So, it's not at all unusual to hear that Hotmail.com is bounding e-mail or routing it to a recipient's spam folder.
Hotmail is now automatically placing messages into user's spam folders if even a single user has marked the content from that user as SPAM in their Hotmail account <RANT> - a bit draconian, I know, but I've never considered Hotmail to be a valid e-mail hosting company and definitely don't recommend them for use as business e-mail.</ RANT>
Hotmail is also checking the headers of forwarded messages very carefully for an SRS FORWARD REWRITE. If not included, they are automatically bounding the message with a "550, REQUIRES AUTHENTICATION" error.
The development team at SmarterMail / SmarterTools is already aware of this problem, and they are working to fix the rare occasion when SmarterMail does not properly insert the SRS Rewrite Header into forwarded e-mail messages, and will include the fix in a minor update in the near future.
NOTE: This occurs only on internal same-server SmarterMail to same-server SmarterMail messages which are forwarded to an external mail server.
So, as CCWH responded above,
There are many, many, reasons why an email can be classed as spam, especially by hotmail.com.
However, you need to speak to your email provider / administrator. SmarterTools can't fix your issue.
with which statement, I concur.
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