Is that "2" really in the log line?
> 1. How can I check, what is wrong with sender address an why it is rejected?
Use sendmail -bt
see the man page, help, or
http://www.sendmail.org/~ca/email/chk-dbg.html
> 2. More important - what should I change in sendmail configuration to accept addresses like above?
Depends on what you find via the testing, but check your access map
for matching entries.
--
Note: please read the netiquette before posting. I will almost never
reply to top-postings which include a full copy of the previous
article(s) at the end because it's annoying, shows that the poster
is too lazy to trim his article, and it's wasting the time of all readers.