derobins wrote:

I forward several Email accounts to Gmail. Now I would like to receive
all these mails from Gmail (POP) but send my mails via the SMTP server
of the original account providers.

When I try to set the POP server to (empty), Outlook does not allow me
to click 'next'.

Why are you trying to set the POP server to a null value? Leave it as it
was and simply disable receiving for the account in the Send/Receive group.
--
Brian Tillman [MVP-Outlook]