ssmtp works in cron, but not elsewhere

Ubuntu 16.04.... If I enter sudo ssmtp [email protected] and type some text, then hit ctrl-d I get in the syslog:

Nov  4 15:18:58 garage-security sSMTP[27717]: 550 5.6.0 Invalid header found (see RFC2822 section 3.6)

But if I look at the message from cron in the syslog, I see

Nov  4 15:01:03 garage-security sSMTP[20685]: Sent mail for [email protected] (221 2.3.0 smtp01.wow.cmh.synacor.com closing connection) uid=0 username=root outbytes=761

Both ssmtp and sendmail fail in the same way. I dont understand why cron can send email but I cant from the command line - don't they both use the same config file? Can someone clue me in?