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Please take %%PREFIX%%/etc/nullmailer/remotes.sample
as an example and edit your own 
%%PREFIX%%/etc/nullmailer/remotes

It's possible refefine mailq and sendmail in your /etc/mail/mailer.conf:

sendmail	%%PREFIX%%/libexec/nullmailer/sendmail
send-mail	%%PREFIX%%/libexec/nullmailer/sendmail
mailq		%%PREFIX%%/libexec/nullmailer/mailq

WARNING! It will couse problem in case delivering to local recipients.
Also man nullmailer-send(8) for more details

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