Allow specific internal/external address to send to a DL - Exchange Online

Hi all,

I need to grant access to one specific internal address and another specific external address to send email to a Distribution List in O365.

I tried to tick the checkbox "allow messages from people inside and outside my organization" then insert both address (the inside and the external) in "Specified senders" but it seems that I cannot insert an external address in "Specified senders"

Doing some researches in the google, I could figureout that I can create transport rules for this case, but thinking about it, if I need to do the same in the future for others DL's I would have a lot of transport rules, that is not good in a management perspective.

So, there is a simple way to proceed allowing only specific internal and external address to send email to a DL?

Thanks in advance