The userPrincipalName field is defined as the “official” user’s email address (and is good for logging in on a workstation too).
With a local AD accounts provider the userPrincipalName value is automatically set in user long name form, i.e. toto@nethserver.org - applying the NethServer domain name.
Instead, the default AD behavior is using the AD domain as suffix (i.e. toto@ad.plop.org). In this case, if you want to set userPrincipalName to a good email address, the configuration must be done manually - or using a Power Shell script or similar.
As alternative you could use the mail field or similar, if you can trust its value. Again, it must be set manually. The field must be also configured to be used by SOGo as well.