GemBox.Email currently supports only ICS format.
The MSG files have their own format for storing calendars, but I’m afraid that GemBox.Email doesn’t support it at the moment.
Because this feature will require quite some time investment, we’ll need to wait for more users to request it. Please note that we prioritize feature request implementations by the number of users requesting them and currently we are working on some other features that have greater priority (more user requests).
Also regarding the line break, it seems that this problem occurs in Microsoft Outlook when the description text uses “\r\n”, but it works fine when using “\n”.
I hope this helps.