Problem with notifications

[mic]

Hello.

This is my beginnings with OTRS system but I want to learn it. So sorry if my question is obvious for you...

I have a problem with messages. When operator send a new message from ticket then customer receive our answer. But operator does not receive messages from system when a new ticket is assigned to him, new actions occured in ticket, etc...

In system log I see:
Fri Feb 7 10:08:17 2014 notice OTRS-CGI-10 POP3: Fetched 1 email(s) from otrs@xxx.pl/mediarecovery.
Fri Feb 7 10:08:17 2014 notice OTRS-CGI-10 FollowUp Article to Ticket [2014020710000028] created (TicketID=5, ArticleID=25). ,
Fri Feb 7 10:08:17 2014 info OTRS-CGI-10 Error sending message
Fri Feb 7 10:08:17 2014 error OTRS-CGI-10 Can't send message: 553Go ahead...
Fri Feb 7 09:57:16 2014 notice OTRS-CGI-10 Sent email to 'test2@xxx.pl,' from 'OTRS System <otrs@xxx.pl>'. HistoryType => SendAnswer, Subject => Re: [Ticket#2014020710000028] TEST;
Fri Feb 7 09:53:31 2014 notice OTRS-CGI-10 Sent email to 'mxxxx@gmail.com,' from 'OTRS System <otrs@xxx.pl>'.

I don't know where I have to set this up
Thanks in advance!
BR, Michael.

[crythias]

Agent preferences Yes, Queue highlighted in my queues.

[mic]

Hi.

Thanks for your answer.
Could you tell me more details about what you wrote ?

Thank you in advance.
BR, Michael.

-----edit-----

OK, I know what are you talking about
Its configures already but notifications still doesn't work system can send Emails but no notifications and I really dont know why

in system log:
Thu Feb 13 09:41:20 2014 notice OTRS-CGI-10 Sent agent 'OwnerUpdate' notification to 'ZZZZZZ@gmail.com'.
Thu Feb 13 09:41:20 2014 info OTRS-CGI-10 Error sending message
Thu Feb 13 09:41:19 2014 error OTRS-CGI-10 Can't send message: 553Go ahead...

Any help ? please...
Michael.

[crythias]

Syconfig, Notification Sender