a weird mail reaching problem in local

[maslakyavuz1]

i have been using a zimbra 8.6.0-GA-1242 for along time. It works. I have a problem 2 times about reaching mail to some local accounts in this week. There is no error about that in zimbra.log and mail.log.
Log files shows the mail arrived in regarding accounts. ( status=sent (250 2.0.0 OK 08/FF-02963-316B47E5) )
As reported to me the mail reached to outside mail address successfully. the mail server has disk space enough.
What can be the problem? May the problem be outlook client or server bottleneck .The server runs in sata disk.

regards