I am not sure about these ones:
2. rejected RCPT Sender verify failed
3. Message is frozen
4. unexpected disconnection while reading SMTP command
I assume #2 means you might have set an invalid bounce or "Send From Email". On some servers the "Send From Email" and bounce address (or, with bounce disabled, the Admin Email) must be a valid mailbox on your account.
#3 means, I believe, that the message(s) could be sent later.
#4 should not really happen. Please let me know if it continues.
Regards