Postfix: find out why mail bounces

My otherwise pretty well working postfix mail server is refusing to deliver a single mail with attachment. Unfortunately i don't have the error message available which was sent to the sender. But here is the relevant part of the log file:

(added line numbers and anonymized some fields)

01 Nov 26 06:57:38 xs.private postfix/smtpd[24013]: connect from aaa.bbb[12.34.56.78]
02 Nov 26 06:57:39 xs.private postfix/smtpd[24013]: 48CFB8BC0CDA: client=aaa.bbb[12.34.56.78]
03 Nov 26 06:57:39 xs.private postfix/cleanup[24029]: 48CFB8BC0CDA: message-id=<1777B66B84D03E42A2A4E0F30A97A2B4385CC85A@X0361.xs>
04 Nov 26 06:57:44 xs.private postfix/smtpd[24013]: disconnect from aaa.bbb[12.34.56.78]
05 Nov 26 06:57:44 xs.private postfix/qmgr[247]: 48CFB8BC0CDA: from=<sender@xx>, size=4785215, nrcpt=1 (queue active)
06 Nov 26 06:57:50 xs.private postfix/smtpd[24047]: connect from localhost[127.0.0.1]
07 Nov 26 06:57:50 xs.private postfix/smtpd[24047]: 6FCC08BC0D66: client=localhost[127.0.0.1]
08 Nov 26 06:57:50 xs.private postfix/cleanup[24029]: 6FCC08BC0D66: message-id=<VSJkfxFXYEjnoK@xs.private>
09 Nov 26 06:57:52 xs.private postfix/qmgr[247]: 6FCC08BC0D66: from=<>, size=6803, nrcpt=1 (queue active)
10 Nov 26 06:57:52 xs.private postfix/smtpd[24047]: disconnect from localhost[127.0.0.1]
11 Nov 26 06:57:52 xs.private postfix/smtp[24034]: 48CFB8BC0CDA: to=<receiver@zz>, relay=127.0.0.1[127.0.0.1]:10024, delay=13, delays=5.5/0.01/0/7.9, dsn=2.5.0, status=sent (250 2.5.0 Ok, id=98796-09, BOUNCE)
12 Nov 26 06:57:52 xs.private postfix/qmgr[247]: 48CFB8BC0CDA: removed
13 Nov 26 06:57:53 xs.private postfix/smtp[24051]: 6FCC08BC0D66: to=<sender@xx>, relay=ccc.ddd[34.56.78.90]:25, delay=2.6, delays=2/0.01/0.28/0.32, dsn=2.0.0, status=sent (250 2.0.0 Ok: queued as 281520DECC8C7371)
14 Nov 26 06:57:53 xs.private postfix/qmgr[247]: 6FCC08BC0D66: removed

In line 11, it says "status=sent", but the BOUNCE in the same line states that the mail is not sent, but bounced.

How can i find out why postfix does not want to deliver this mail?

In line 9, the sender changed from "sender@xx" to "", could this be the reason? If so, why did the sender address disappear?

Some more info: This is the mail system of a Mac server, so this is coming more or less out of the box, at least i haven't done any configuration changes..

And btw, can you recommend a good and up-to-date book about postfix and dovecot?