mIRC 7.32 Windows 8 64bit.
Tools - Options... - Connect - Email type 'FirstSecond@Example.Com', in Identd - User ID type 'Third', untick 'Use ID from email address', connet to an IRC server, your ident is 'FirstSecon', as in 'Nick!FirstSecon@Host.IP', here
I found this
mIRC should use your email address (as a first option - if it is not set, mIRC defaults to your identd userid and if that is not set it uses a default value) in the USER message
shouldn't mIRC use User ID 'Third'? It seems more logical to me.
At the moment you cannot set an email address and use a different ident the way I described, you have to remove the email address to use 'Third' as ident. I know I can use /server -i.