Outlook 2016 on WIN 10 2019
PopUp warning eliminated by setting encryption to NONE per Homestead Setup at :
http://websitehelp.homestead.com/SBMaster/Content/EmailIMAPSettings.htm
QUESTION -> Is this a safe setting ?
Other Homestead recommended encryption setting is TLS, unfortunately this is not an option in Outlook 2016.
Only encryption settings are SSL/TLS , STARTTLS, AUTO - all of these cause an Internet Security Warning PopUp when enabled.
>>pete
Contact your email provider, Outlook is not related to Roundcube.