I've just installed EPiServer Mail into an EPiServer CMS 5 SP2 site that is running under .NET 3.5 (yes i know its not officially supported but hey). Much to my annoyance I had 3 errors to fix - which simply meant removing 3 lines from the web.config that either the EPiServer Mail install (or the hotfix) added. The offending lines which must be removed are as follows:
<add assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
<add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />