Thank you for your post.
From FileMaker Pro Help, Error 1506 is "Email(s) could not be sent successfully". Check the SMTP server error logs to determine why the could not be sent. If the "input data is the same", what occurs if you try sending it manually? That is, pull down the File menu, select Send -> Mail... and manually put in the data. Does this work? If this works, try creating a new script with the same information as the manually entered data and execute the script. Does this work?
Keep me updated with any progress.