External Email from Newsletter Mailing Fails
If you're still interested - what version of Civi are you running? And you have selected PHP mail() as your mailer so it is odd that sendmail is referenced in the error msg. 
I can just say that I have Civi 4.7.20 (for WP though I think that isn't relevant). System outbound mail is set to mail().  I only got email to work by adding back check for blank CC/BCC that was in older versions of CRM/Utils/Mail.php
'Save and send test email' works b/c it doesn't go to Utils/Mail it just instantiates its own PEAR Mailer and sends a header with just 'to','from''subject' and 'message'