Hello Takayuki Miyoshi,
I have created a form with the possibility of selecting the sender to send the message, I look at this site created by me (link).
The site of the link was posted by a HOST online and so I think that uses the library PHPMailer().
In my case I will publish the site via a local server, using Wampserver, so I'm configuring the SMTP submission.
If I set a single destinarario, without drop-down menu, sending works.
If I insert the drop-down menu to select the recipient by sending error and no longer works.
Thanks!