DNN 9.9.0 includes the new MailKit provider for better support for Office 365. 

Mandeeps Common Library has been leveraging MailKit provider for a while now and hence there is a version conflict if you upgrade to DNN 9.9.0

Recommendation

Be sure to upgrade to latest version of Mandeeps Common Library (v4.0.6 as of this post) prior to upgrading to DNN 9.9.0

If you do not upgrade to the latest Mandeeps Common Library and attempt an upgrade to DNN 9.9.0 then it's likely that your site will throw an error indicating a version mismatch of MailKit and MimeType assemblies. You can quickly resolve this error by simply copying both MailKit.dll and MimeKit.dll from Mandeeps Common Library Install Package to your sites' /bin folder.