I have all my Debian systems with the default exim4 MTA configured as a "satellite system" of my ISP's outgoing mailserver (I stay well away from sendmail after previous experiences on RedHat). I have no problems using reportbug or anything else which expects to be able to send mail via a service on localhost. However, it does mean things like cron logs make a roundabout trip via my ISP's POP3 mailbox rather than just being locally delivered... but it works for me.
(If you want to explore this route, during
dpkg-reconfigure exim4-config, the relevant option for "General type of mail configuration" is "mail sent by smarthost; no local mail" and later give the name of the ISP's SMTP server for "outgoing smarthost".)