* MessageCompose now adds message-id to new messages (it was previously
done on its behalf by MimeMessage).
* LegacyConversions.updateMessageFields() now handles missing message-id
without error.
* Unit tests for the LegacyConversions change
These two issues were combining with a failure of comcast's SMTP server
to insert message-id headers, to prevent delivery of a message between
any two comcast accounts using this client.
Bug # http://b/issue?id=2161478
eb7752bf69