Outlook on no domain users

Hi Guys,

my scenario is as next ,

have exchange picking up emails using pop connector from our ISP domain registra.

we are sending using smarthost of isp.

we have already a configured webmail.domain.com and everything works fine .

we have some users which are not member of the domain using office 2013 ,

we want to add their mailbox to their outlook using outlook anywhere.

can you please advise how to do so ? should I enable some feature on the exchange side to make this happens ?

thank you