Postgres Out of Memory

Hello

When I run the following query, I have the error "Out of memory". I
believe that is memory problem, but before changing any parameter in
Postgres configuration files I remember that under ~data/pgsql_temp/
Temporary files used to appear when I ran a big query each temporary was
approximately about 1 gigabyte.

Here is my query

SELECT proceso, id_mail, to_mail, temp
FROM (
SELECT proceso, id_mail, to_mail, count (*) AS temp
FROM SENDMAIL2
GROUP BY proceso, id_mail, to_mail
HAVING proceso = 'Automatas'
) AS TAB_TEMP
HAVING temp != 1

The output of Explain is here

QUERY PLAN
Subquery Scan tab_temp (cost=622577.68..622612.73 rows=2003 width=118)
-> HashAggregate (cost=622577.68..622592.70 rows=2003 width=110)
Filter: (count(*) <> 1)
-> Seq Scan on sendmail2 (cost=0.00..622327.31 rows=20029
width=110)
Filter: ((proceso)::text = 'Automatas'::text)

My /etc/system file is this
set shmsys:shminfo_shmmax = 1073741824
set shmsys:shminfo_shmmin = 1
set shmsys:shminfo_shmmni = 100
set semsys:seminfo_semmni = 100
set shmsys:shminfo_shmseg = 10
set semsys:seminfo_semmsl = 100
set semsys:seminfo_semmns = 2500

set semsys:seminfo_semopm=10
set semsys:seminfo_semaem=16384
set semsys:seminfo_semume=10
set semsys:seminfo_semmap=1
set semsys:seminfo_semusz=96
set semsys:seminfo_semmnu=30
set semsys:seminfo_semvmx=32767
forceload: sys/semsys
forceload: sys/shmsys

My postgresql.conf is this
shared_buffers = 50000
work_mem = 2097151
maintenance_work_mem = 2097151

Please help me!!!

Thanks in advanced

--
If I have seen further it is by standing on the shoulders of the giants
Isaac Newton