According to the man page 

"The masks specified to the mget and mput commands act as filters for 
directories rather than files when recursion is toggled ON. 

The mask specified with the mask command is necessary to filter files within 
those directories. For example, if the mask specified in an mget command 
is "source*" and the mask specified with the mask command is "*.c" and 
recursion is toggled ON, the mget command will retrieve all files 
matching "*.c" in all directories below and including all directories 
matching "source*" in the current working directory. 
"

However, looks like mask command is not filtering the files:

$ /opt/datareturn/samba/bin/smbclient -V
Version 3.0.2a
$ cd /tmp
$ pwd
/tmp
$ ls -lR tst*
tst1:
total 32
-rw-r--r--   1 ruslan   other         29 Apr  6 16:57 1.gz
-rw-r--r--   1 ruslan   other         29 Apr  6 16:57 1.zg

tst2:
total 32
-rw-r--r--   1 ruslan   other         29 Apr  6 16:57 2.gz
-rw-r--r--   1 ruslan   other         29 Apr  6 16:58 2.zg
$ /opt/datareturn/samba/bin/smbclient '//1.253.1.32/test' -U ruslan%password
Domain=[GONDOR] OS=[Windows 5.0] Server=[Windows 2000 LAN Manager]
smb: \> recurse on
smb: \> mask *gz
smb: \> mput tst*
Put directory tst2? y
Put file tst2/2.zg? n
Put file tst2/2.gz? n
Put directory tst1? y
Put file tst1/1.zg? n
Put file tst1/1.gz? n
smb: \> quit

As you can see, mput picks up not only *gz files, but *zg also. I've tried the 
same commands on RedHat Linux running 2.2.7 and got the same problems.

Thanks!