postfix status=sent but not delivered
i'm trying to send a newsletter, this is my postfix mail.log
postfix/smtp: AD9EA6FC9B1: to=<xxxx...@xxxxxx.it>,
relay=mail.libero.it[188.8.131.52]:25, conn_use=292, delay=27239,
delays=24040/3198/0.01/0.98, dsn=2.0.0, status=sent (250
<492C05DA010431DA> Mail accepted)
only a fews thousand mail were sent:then mail.log always says status=sent, Mail accepted but nothing being delivered.
Even if i try to send myself a preview of that specifical mail its not delivered, but with a new one (different content), it works correctly.
Any hint/ideas please? might it be a spam filter on mail dimension or what?