must be in root group

[info NTS]

I use sendmail fine, but all my users need to be in the "root" group unless
the account doesn't work : cant receive , send & check ...

the server answer (i put my user in 'mail' group)

 when i try to send a mail to it :

   ----- The following addresses had permanent fatal errors -----
<account na domainname.com>
    (reason: can't create (user) output file)
   ----- Transcript of session follows -----
procmail: Lock failure on "/var/spool/mail/info.lock"
procmail: Error while writing to "/var/spool/mail/info"
550 5.0.0 <account na domainname.com>... Can't create output

 when i try to check it say :

error in mail server connection. password refused. account : 'account name',
Serveur : 'ip-adress', Protocole : POP3, server answer : '-ERR System error,
can't open temporary file, do you own it?', Port : 110, Secure (SSL) : No,
server error: 0x800CCC90, erreur number: 0x800CCC92

Thanks for help.