The avast splash screen is just telling you that the plugin is functioning. Outlook express should be loading after that, just as it did before you installed avast.
No, not at all. The splash screen is shown because Outlook Express is not being invoked. Terminating the Outlook/Exchange provider will make no difference.
What are you seeing?
You are not seeing an avast plug-in taking over. The splash screen you are seeing is the avast plug-in for Microsoft Outlook being loaded by Outlook. It is Microsoft Outlook on your system that has taken over the job of being the program invoked by the "Send to mail recipient".
Why has this happened?
Not because you installed avast for sure. I believe this happened because you have Microsoft Outlook installed on your system (perhaps as part of Microsoft Office) and this month there was an update to Microsoft Outlook installed by the monthly Windows update. When there is an update to Microsoft Outlook the Windows update mechanism has a really annoying habit of again making Microsoft Outlook the default email client on your system. So Microsoft Outlook has jumped in and taken over even though you do not want it to. This certainly happened on my system and I had to correct it.
How to fix it?
I believe that if you go to Windows Control Panel > Internet Options > Programs tab
you will find that the E-mail program is set to "Microsoft Office Outlook". In the drop down list please select "Outlook Express" click on "Apply" and "OK". Then restart your system. I hope you will find that "Send to mail recipient" works again as you want it to.