title: Creating an ICS file in python sends as a plain text email to external clients
body: <p>I'm creating an ICS to attach to an email with SMTPLib and Email libraries in python. When I send the meeting request, it shows as a meeting request normally should inside of our organization. When I send the meeting request to people outside of the organization, it shows to those people as just a plain text email with no options to accept, decline, etc.</p>
  
  <p>Pasted below is the output are the email headers being received which includes the icalendar format. Many of the possible solution I've found are that it's an error on the user's end but when I send a meeting request created in outlook and send it to an external email address, it will work fine.</p>
  
  <p>Is my icalendar format incorrect?</p>
  
  <pre><code>Content-Type: multipart/mixed; boundary
  MIME-Version: 1.0
  Reply-To: test@test.com
  Date: Fri, 15 Jun 2018 10:55:17 -0400
  Subject: test
  From: test &lt;SchedulerApp@test.com&gt;
  To: test@test.com,another.test@gmail.com
  
  Content-Type: multipart/alternative; boundary="
  
  MIME-Version: 1.0
  
  
  Content-Type: text/html; charset="us-ascii"
  MIME-Version: 1.0
  Content-Transfer-Encoding: 7bit
  
  &lt;h1&gt;Test Email Body&lt;/h1&gt;
  
  MIME-Version: 1.0
  Content-Type: text/calendar; method="PUBLISH"; charset="us-ascii"
  Content-Transfer-Encoding: 7bit
  
  BEGIN:VCALENDAR
  PRODID://SchedulerApp//test company//en
  VERSION:2.0
  CALSCALE:GREGORIAN
  METHOD:REQUEST
  BEGIN:VEVENT
  DTSTART:20180616T074500
  DTEND:20180616T084500
  DTSTAMP:20180615T105517Z
  ORGANIZER;CN=SchedulerApp:mailto:test@test.com
  UID:FIXMEUID20180615T105517Z
  ATTENDEE;CUTYPE=INDIVIDUAL;ROLE=REQ-    PARTICIPANT;PARTSTAT=ACCEPTED;RSVP=TRUE
   ;CN=test@test.com;X-NUM-GUESTS=0:
   mailto:test@test.com
  ATTENDEE;CUTYPE=INDIVIDUAL;ROLE=REQ-    PARTICIPANT;PARTSTAT=ACCEPTED;RSVP=TRUE
   ;CN=another.test@gmail.com;X-NUM-GUESTS=0:
   mailto:another.test@gmail.com
  CREATED:20180615T105517Z
  &lt;h1&gt;Test Email Body&lt;/h1&gt;
  SEQUENCE:0
  SUMMARY:test
  LOCATION:test location
  TRANSP:OPAQUE
  END:VEVENT
  END:VCALENDAR
  </code></pre>