I wish to check the email address to ensure that users are registering with an email from a particular domain
Can someone please tell me how and if this can be achieved using ADDT and PHP?
Yes it can be done. At the end of most of the ADDT wizards is the ability for validation. Typically when choosing one for email you would choose email address, but this would do firstname.lastname@example.org since you want it to be specific you need to choose Regular Expression then add your expression to the field.
Something like this would work as a regular expression.
More on regular expressions at:
That's brilliant - thank you very much for your help. The web site you recommended is also very useful.