… although not strictly related, I notice that sendmail is totally disregarding whatever email address I use on the "from" field, as well as the "fromName", falling back to the username under which Cells is running (and, if set, the full name of the user, as it appears under /etc/passwd). But this might be the intended behaviour, I don’t know!
There is also a tiny bug from the test panel on the UI (probably something that will be addressed once v5 is launched!) — when adding just one external address, the email will come with a comma as part of the address, e.g. To: <my-valid@external.address, > — note the comma glued to the TLD part! — which, of course, is invalid, and will be immediately rejected by my MTA (Postfix, in my case), even if that address is a local mailbox (just not one associated with any user).