Sends a message.
The "fileHandleId" field should point to a plain text file containing the body of the message. The file should be uploaded prior to this call.
The "recipients" field should hold a set of IDs corresponding to the recipients of the message.
All other fields are optional, including the "subject" field. To chain messages together into a conversation, specify the message you are replying to via the "inReplyTo" field.
In most cases, message delivery is asynchronous to message creation. i.e. It may take several seconds for a message to appear in a user's inbox.
Unauthorized delivery, such as messaging a team you are not affiliated with, will result in a bounce message being sent to your email.
There are limits on the number of message recipients you can specify (50) and the rate at which you can send messages (10 per minute). Neither these restrictions, nor the restriction that you can't message a Team with which you are unaffiliated, apply if you are a member of the Trusted Message Senders Team.