We're using the new gmail api on iOS to send emails and everything works great for messages with single recipients. When we specify more than one in the "to" field, we get the following error:
Error Domain=com.google.GTLJSONRPCErrorDomain Code=400 "The operation couldn’t be completed. (Invalid to header)
I have verified the content we are sending is in fact a valid rfc822 message.
I had an exchange with the gmail team and they did confirm that this is actually a bug with their api. Not sure when it will be fixed as they didn't provide any more details but it's on their radar.