drwxrws---  2 smmsp smmsp 4096 May 25 22:47 mqueue-client/

usermod -a -G smmsp www-data

# groups www-data
www-data : www-data smmsp

sendmail[13152]: NOQUEUE: SYSERR(www-data): can not chdir(/var/spool/mqueue-client/): Permission denied