Exim – Sender verify failed – rejected RCPT

While checking on Exim's log messages I found many entries of the following message
"Sender verify failed" "rejected RCPT" …
I 'm not an exim expert… I'm afraid Exim is not delivering 100% emails to recipients, because our Email Marketing Application its getting a lower OPEN RATE.

Can someone helpe understand this log messages?
Is it my server saying "No Such User Here" or a remote server?
174.111.111.11 represents my server IP.
Thanks

Exim log

2010-10-02 14:00:19 SMTP connection from myserverdomain.com.br () [174.111.111.11]:54514 I=[174.111.111.11]:25 closed by QUIT
2010-10-02 14:00:19 SMTP connection from [174.111.111.11]:54515 I=[174.111.111.11]:25 (TCP/IP connection count = 2)
2010-10-02 14:00:19 H=myserverdomain.com.br () [174.111.111.11]:54515 I=[174.111.111.11]:25 Warning: Sender rate 672.4 / 1h
2010-10-02 14:00:19 H=myserverdomain.com.br () [174.111.111.11]:54515 I=[174.111.111.11]:25 sender verify fail for <fernandagaglianone=oi.com.br--2723--bounce@e-mydomain.com.br>: No Such User Here
2010-10-02 14:00:19 H=myserverdomain.com.br () [174.111.111.11]:54515 I=[174.111.111.11]:25 F=<fernandagaglianone=oi.com.br--2723--bounce@e-mydomain.com.br> rejected RCPT <fernandagaglianone@oi.com.br>: Sender verify failed

2010-10-02 14:00:19 SMTP connection from myserverdomain.com.br () [174.111.111.11]:54515 I=[174.111.111.11]:25 closed by QUIT
2010-10-02 14:00:19 SMTP connection from [174.111.111.11]:54516 I=[174.111.111.11]:25 (TCP/IP connection count = 2)
2010-10-02 14:00:19 H=myserverdomain.com.br () [174.111.111.11]:54516 I=[174.111.111.11]:25 Warning: Sender rate 673.3 / 1h
2010-10-02 14:00:19 H=myserverdomain.com.br () [174.111.111.11]:54516 I=[174.111.111.11]:25 sender verify fail for <littlepet.chaves=gmail.com--2723--bounce@e-mydomain.com.br>: No Such User Here
2010-10-02 14:00:19 H=myserverdomain.com.br () [174.111.111.11]:54516 I=[174.111.111.11]:25 F=<littlepet.chaves=gmail.com--2723--bounce@e-mydomain.com.br> rejected RCPT <littlepet.chaves@gmail.com>: Sender verify failed

2010-10-02 14:00:19 SMTP connection from myserverdomain.com.br () [174.111.111.11]:54516 I=[174.111.111.11]:25 closed by QUIT
2010-10-02 14:00:19 SMTP connection from [174.111.111.11]:54517 I=[174.111.111.11]:25 (TCP/IP connection count = 2)
2010-10-02 14:00:19 H=myserverdomain.com.br () [174.111.111.11]:54517 I=[174.111.111.11]:25 Warning: Sender rate 674.3 / 1h
2010-10-02 14:00:20 H=myserverdomain.com.br () [174.111.111.11]:54517 I=[174.111.111.11]:25 sender verify fail for <Luciene_souza_vasconcellos=hotmail.com--2723--bounce@e-mydomain.com.br>: No Such User Here
2010-10-02 14:00:20 H=myserverdomain.com.br () [174.111.111.11]:54517 I=[174.111.111.11]:25 F=<Luciene_souza_vasconcellos=hotmail.com--2723--bounce@e-mydomain.com.br> rejected RCPT <Luciene_souza_vasconcellos@hotmail.com>: Sender verify failed