And as a reminder (since I just experienced this myself), if you get
an error message like:
"The message's content type was not explicitly allowed"
Change your email client to send text/plain.
For example, in Gmail, you have to go into Settings (under the gear
icon menu in the top right), and at the bottom, choose:
Outgoing message encoding: (*) Use default text encoding for outgoing messages
In addition, if you're using the recently "new" Gmail "Compose"
feature, you may have to click the little drop-down arrow button menu
to the bottom right of the compose text area, and make sure that
√ Plain text mode
is checked.
Thanks,
Tantek