Use a test SMTP service or infrastructure you are authorized to inspect.

Follow the session:

220 greeting
EHLO client.example
250 capabilities
MAIL FROM:<sender@example.com>
250 accepted
RCPT TO:<recipient@example.net>
250 accepted
DATA
354 send content
...
250 queued
QUIT

Identify where STARTTLS occurs, whether authentication is appropriate for that service, and which response belongs to each command.

Then deliberately use an invalid recipient on your own test environment and compare the response.

Reference material