Our system has been running Samba-3.6.6 since mid-2012 without great troubles.

Recently we upgraded Samba to 4.1.3 and found that the file locking mechanism seemed to prevent email delivery when the inbox was kept open by the mailer on WinXP/Win7 clients.

Server environment : Linux kernel 3.8.8; GLibC-2.18; Python-2.6.8
Samba-4.1.3 build options : --with-quotas --prefix=/10/samba-4.1.3 --datarootdir=/10/samba-4.1.3
Samba-3.6.6 build options : --prefix=/10/samba-3.6.6 --datarootdir=/10samba-3.6.6 --enable-shared --with-quotas --with-sys-quotas
smb.conf : (see attachment)
MTA : sendmail-8.14.6
Clients OS : WinXP/Win7
MUA : PC-Pine 4.64
INBOX path : "~user/.mail/inbox" on server, "X:\.mail\inbox" on clients

Steps to reproduce error :
1. map home directory under X: "net use X: \\server\username"
2. invoke PC-Pine and open INBOX (see attached "step2.jpg")
3. send new message to "username@domain"
4. refresh list of messages, Samba-4.1.3 shows no change, Samba-3.6.6 works correctly (see attached "step4-3.6.6.jpg")
5. quit PC-Pine
6. invoke PC-Pine and open INBOX (see attached "step6-4.1.3.jpg" and "step6-3.6.6.jpg)
7. send a further message to "username@domain"
8. refresh list of messages, Samba-4.1.3 shows no change, Samba-3.6.6 works correctly (see attached "step8-3.6.6.jpg")
9. disconnect drive mapping "net use X: /d"

The message sent in step 3 was written to INBOX, just not shown on refreshing,
but the one sent in step 7 had never been written to INBOX, hence the mail was lost.

Attached please also find the level 10 Samba logs of the sessions of steps 1-9.