Is there any way how to delete some mails from mailbox on the server ?
Is this possible track send mail ?

Edit#
This may be important:
each of mail accounts has a file /var/mail/loginname

login name looks like plain text file where i can read who is the sender and the body of the msg.

So it looks like this is a place where mails are stored. Is it good idea to edit this file to remove some too big e-mails ?

If you want to remove certain messages from a mailbox, you should do it using an MUA, not a text editor. Most distros have a command-line MUA which is invoked by typing "mail" (if you are root and you want to fool with someone else's mailbox, type "mail -u username" instead). You'll see a list of numbered messages, followed by a "?" prompt. To delete message 5, for example, type "d 5". Repeat as needed. When done, type "q" to quit. If you made a mistake, type "x" instead of "q" to leave everything unchanged. "man mail" for the complete story.

Only to the next MTA. If you're using slackware, the transaction is logged in /var/log/maillog

command mail -u username works perfect.
There is another problem.
How to achive this same ( deleting some mails from mailbox on server ) when mailbox is stored in directory.
Structure is like that:

/home/company/user1/Maildir/
in Maildir i can find /New
/Cur
/Tep

In /New i can find files like 1121767625.25528_0.tubka
This files look like text file where i can read who is the sender and the body of the msg.

Try something like I don't know for sure.