I really have no idea since that is a fairly old version.
What I can tell you is in March of 2014 a new EmailValidator control was added in mojoPortal code and is currently used on both the login and registration.
This was added because there are some known issues with denial of service that can be worked against server side regex validation.
So this new EmailValidator inherits from RegularExpressionValidator but overrides the EvaluateIsValidMethod
Since it inherits from RegularExpressionValidator it does use regex client side but by default it uses a different method server side to validate email. What it does is new up a new instance of System.Net.MailAddress using the provided string an if an exception is trapped it is not valid.
There is a new configuration setting which can override this and make it use regex server side.
<add key="ForceRegexOnEmailValidator" value="true" />
but it still uses regex in client side validation no matter what.
There is also a new config setting that could possibly be used to override the regex expression:
<add key="CustomEmailRegex" value="" />
if that has anything other than empty string it will use that as the regex expression. However, I don't know if this has been actually tested. My worry would be that regex expressions may possibly have characters that need to be escaped in xml when adding the setting. I don't know if that is a problem or not.
now all of this information is based on reviewing the latest code. I really have no idea or suggestions for older versions. I don't know how long that CustomEmailRegex setting has been there, it might have been there for quite a while but it might also have been added around the same time as the EmailValidator control.