Blocking outgoing mails to some users in exim

You can use the following acl

acl_smtp_rcpt = acl_check_rcpt

acl_check_rcpt:

  deny
     message = The $sender_address is prohibited to send mail to the $domain
     senders = lsearch;/etc/exim/restricted_sender
     domains = lsearch;/etc/exim/restricted_domains

/etc/exim/restricted_sender
[email protected]

/etc/exim/restricted_domains
gmail.com

Testing

# swaks -s mail.example.net --to [email protected] --from [email protected]
=== Trying mail.example.net:25...
=== Connected to mail.example.net.
<-  220 mail.example.net, [xxx.xxx.114.28]
 -> EHLO www.example.net
<-  250-mail.example.net Hello www.example.net [xxx.xxx.114.28]
<-  250-SIZE 52428800
<-  250-PIPELINING
<-  250-STARTTLS
<-  250 HELP
 -> MAIL FROM:<[email protected]>
<-  250 OK
 -> RCPT TO:<[email protected]>
<** 550 The [email protected] is prohobited to sent mail to the gmail.com
 -> QUIT
<-  221 mail.example.net closing connection
=== Connection closed with remote host.