Domain is a read-only String property that indicates the host name or mail exchange where the email will be delivered using a delivery mechanism as described in the Internet Standards document: 

RFC 2821 - Simple Mail Transfer Protocol  

Domain returns the contents of the Address property following the "@" symbol that indicates the beginning of the domain literal for the address. 

Domain can contain an empty string ('') when Address does not contain a value, or the "@" symbol cannot be located in Address. 

Use Address to access non-domain information (like comments) that may be included in the email address follwing the value in Domain. 

Use Name to access the optional name of the person for the email address. 

Use User to access the mailbox name for the email address. 

Use Text to access the Name and Address concatenated into an RFC 2822-compliant form.

TIdEMailAddressItem.Address 

TIdEMailAddressItem.User 

TIdEMailAddressItem.Name