In short,
It is not possible to have different AD_domain- and ActiveSync_mail_domain-name with our configuration. 
The main underling reasons are:
- For user account information SOGo exclusively relies on the content of the accounts provider e.g. user account content of the Active Directory. SOGo does not support (internal) identities resulting out of design philosophy user accounts shall be administrated on a central place.
- Aiming to be a single office solution Nethserver does not store email aliases in the AD account provider. To be accurate it does not store any email information; The userPrincipalName, defaulting to username@AD_domain, is used instead.
- ActiveSync is a single account solution and does (AFIAK) not support identities, This does not mean it can’t use a different domain-names for user -account and e-mail. As said above our configuration does not provide this.
Although your wish/demand does not stand on itself, it turned out to be a complex functionality.
The tree grows quickly if one goes through to all possible configurations and exceptions one need to catch if this functionally would be implemented. It’s like a pandoras box which stayed close. 
This being said,
I do not think it is completely impossible if you accept to manage your user accounts with other tools then Nethserver offers. (like M$ RSAT). It may be necessary to change your AD-schema to be able to store e-mail (aliases). If all is set you may use the CustomEmailField setting of nethserver-sogo to use an alternative email address field (AD-entry).
I do not know if someone actually succeed doing this kind of setup and it does not appeal to me to manage user accounts with another tool.