[CentOS] Centos 7 rsync problem

Hello list.

I have a problem with rsync on a FUJITSU PRIMERGY TX150 S7 mailserver
with Linux version 3.10.0-862.11.6.el7.x86_64. The server have a
hardware raid 5 megaraid_sas and xfs partitions.

I have an external network disk (mounted as cifs) where we backup emails
from old centos 6 mailserver and files from also centos 6 file server.

On the new mail server with centos 7 (full updated) the rsync starts but
stop transferring files after some minutes and disappears from Top but
service is alive on ps aux.

The email files (cyrus imap files) are mostly small files.

Unfortunately there is no error so I cant understand where is the
problem. We try an external usb (ext3) and transfer 40Gb emails without
problem.

We give the command:

rsync -vaR --delete --log-file=/var/log/rsync /var/lib/imap
/var/spool/imap/ /mnt/backup/mailserver/

Any ideas?

Thank you in advance.