Aha - I've just come here after being in Slack where I saw same question My comment there was:
This is a sample cURL options calc that works for me with Mailgun… note ‘special’ character used in front of the $emailHTML variable. I also substitute all pilcrows (¶) in the $emailHTML variable with Char(10)…
"-X POST
--user \"api:key-1234567890XYZ0987654321\"
-F \"from=" & $fromEmailAddress & "\"
-F \"h:Reply-To=" & $replyToEmailAddress & "\"
-F \"to=" & $recipientEmailAddress & "\"
-F \"subject=" & $emailSubject & "\"
-F \"html=<$emailHTML\"
-F \"text=" & $emailText & "\"
-F \"attachment=@$myfile;filename=" & $myfilename & "\"
"