I am not sure to correctly understand what kind of split testing (AB testing ?) you are trying to do with the email/address, sorry.
Do you need to split with the domain name part (in that case you should use message.@domain) ? or testing if it is an email format or a phone?
For your delivery personalization, you want to use <%= message.address %> in your subject field.
This message.address is actually the element @address of deliveries logs (nms:broadLogRcp).
With default target mapping (using nms:recipient as factory mapping), and for the channel Email, the recipient.email is copied into broadLogRcp.address during emailing delivery preparation. If the channel SMS (Texto) is used, then the mobile phone attribute of recipient is used for copying the value into broadLogRcp. Same for other channels, the attribute taken depends on them. So take care of type of deliveries when dealing with the address field.
Maybe you would test the channel with <%= message.delivery.messageType %>.
Well, perhaps I totally misunderstood your issue, so please give some details with the error message for some accounts? Is is an empty value?