Mandrill "reject_reason": "invalid-sender"

It'd be useful to see the call/email that's being sent. That error means that there's an invalid sender, as indicated in the reject reason field. That could be because of an invalid email address, invalidly-encoded from name, or invalid or broken encoding in other headers making it so that Mandrill can't parse the "from" header, but without seeing the actual email that you're sending, it's hard to say for sure exactly what the issue is.

You probably want to check that there's a fully-qualified domain name in the from email address, and that if the subject line is encoded, there aren't things like newline (\n) characters that break multibyte characters in the subject line. If you aren't able to identify the issue in the raw SMTP message, feel free to get in touch with support for further troubleshooting assistance.

I had the same problem, in my case, I had forgotten to complete the template defaults "From Name" and "Subject".

I had the same problem. In my case encoding in headers was the problem. I did change the headers encoding to UTF-8 and it worked. I was using C# SMTP and the code is below.

message.HeadersEncoding = Encoding.UTF8;

Hope it works!