In essence Nethserver does not offer a multi tenancy mail-server. Althoug with the offered mail-aliases you can come close to this behavior. It’s is discussed in various posts how to set this up, here a recent one:
Note all solutions rely on a mail-client which has “entities support” to be able to “send from”
(AFAIK TB and roundcubemail are among them)
Although it is in the “needs work” category I recommend to install the nethserver-mail(2) module. Actually it should be in the non existing “work has been done” category. Only mail(2) offers DKIM support. (to be thorough) As discussed before: antivirus has high demand for memory, 1GB comes a little bit to short.
I’m not sure if its a necessity to give clamd (antivirus) more time to startup for armhfp, if this is the case we may implement this:
The nethserver-roundcube module works like a charm. I recommend to setup the mail-server and add nethserver-roundcubemail to it afterwards.