=================================== Email Forwards in MySQL
forwarding router - which even supports multiple addresses (so an
addy can forward out to more than one final recipient):

virtual_user_fwd:
        driver                  = redirect
        verify                  = no
        check_ancestor          = yes
        hide_child_in_errmsg    = yes
        domains                 = +domain_virtual
        data                    = ${lookup mysql{SELECT fwd FROM userforward
WHERE host = '${quote_mysql:$domain}' AND user = '${quote_mysql:$local_part}' AND fwd IS NOT NULL} {${sg{$value}{\\n}{, }}}}

Not sure if you want hide_child_in_errmsg - I was just testing that out...
It hides the final recipients if a bounceback is generated (try with it
on/off to see the difference).
=================================== SMTP auth, MySQL & passwords
MYSQL_AUTH_CRAM = SELECT MYSQL_SMTPAUTH_PASS_FIELD from
MYSQL_SMTPAUTH_TABLE where MYSQL_SMTPAUTH_USER_FIELD = '${quote
_mysql:$1}' AND MYSQL_SMTPAUTH_PASS_FIELD != ''

cram:
        driver = cram_md5
        public_name = CRAM-MD5
        server_secret = ${lookup mysql {MYSQL_AUTH_CRAM}}
        server_set_id = $1
--------------------------
mysql_cram:
  driver = cram_md5
  public_name = CRAM-MD5
  server_secret = ${lookup mysql{SELECT password FROM users \
                  WHERE user = '${quote_mysql:$1}'} {$value} fail}
  server_set_id=$1
--------------------------
domainlist local_domains = \
  ${lookup mysql {SELECT domain FROM domains \
    WHERE type="local" and domain="${domain}" }}


# Never do _local_ delivery to these users.
never_users = root

# Name lookups
host_lookup = *
rfc1413_hosts = *
rfc1413_query_timeout = 30s

mysql> select * from domains ;

domain              type
+-------------------+--------+
|mydomain.com       | local  |
+-------------------+--------+
=================================== 
>Is it possible to make just a query and then instruct exim to get the
>data from the yielded row ?

for ACLs you could do:

set acl_m0 = ${lookup mysql {select f1,f2,f3 from table where f4='x'}}

and ${extract {f1}{$acl_m0}} will return the data of the first field.
In routers you can use address_data to do the same thing.

Check:
http://www.exim.org/exim-html-4.30/doc/html/spec_11.html#IX784
and
http://www.exim.org/exim-html-4.30/doc/html/spec_9.html#SECT9.17
===================================
domainlist blocked_domains = mysql;select 1 from blockeddomains where \
                               domain = '${quote_mysql:$sender_domain}'
=================================== block by IP
block list in a MySQL database.  We reject
at RCPT time, with a message which is mostly a URL to a web page which gives
the other end a chance to request exemption (which of course some annoying
mail programs will refuse to show the sender).

The database has a column for "network" (meaning /24) and for host...if host
is NULL the whole subnet is blocked, else just the specified IP.

This rule is in the SMTP time ACL (ie, once per connection):

  warn    hosts     = +rbl_hosts : !127.0.0.0/8 : !+local_networks :
!+relay_from_hosts
          condition = ${if ! match
{$sender_host_name}{\Nmx\.aol\.com$\N}{yes}{no}}
          condition = ${lookup mysql{select id \
                               from blocklist \
                               where
network=SUBSTRING_INDEX('$sender_host_address', '.', 3) \
                                and
(host=SUBSTRING_INDEX('$sender_host_address', '.', -1) or \
                                     host is NULL) \
                               limit 1 \
                        }{yes}{no}}
          set acl_c1 = blocklist

Later at RCPT time, if an exemption mechanism hasn't changed acl_c1, we find
  deny
      condition = ${if eq {$acl_c1}{blocklist}{yes}{no}}
      message   = rejected see
http://www.olympus.net/local/$sender_host_address