This expression works perfectly well for me. Here is what it looks like inside the expression validator: 

<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ControlToValidate="txtSenderEmail_" Display="Dynamic" ErrorMessage="Email address is invalid" ValidationExpression="^[_a-zA-Z0-9-]+(\.[_a-zA-Z0-9-]+)*@[a-zA-Z0-9-]+(\.[a-zA-Z0-9-]+)*\.(([0-9]{1,3})|([a-zA-Z]{2,3})|(aero|coop|info|museum|name))$"></asp:RegularExpressionValidator>

[edit]

You can also use this one here - really basic one, though:              \w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*

Hi,

I've tried adding some static test to the user.ascx file to see if the site was indeed looking at the file was editing.  I put 'TEST' on the page somewhere and it didn't appear on my portal.  So I think I'm not editing the correct file.

Hi,

That is the file I'm editing....Don't know why the change is not being 'seen'.  Going to try to edit the web.config file to force a restart, empty cache and see what that does.

EDIT:

Okay i've tried the above and email validation still doesn't work.  This is really starting to get frustrating.  Maybe I should just upgrade 4.5.5.