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I have a bash script that I use with nagios that sends notifications by email. The key part of it looks like this:

# $1 (FROM) | $2 (TO) | $3 (SUBJECT) | $4 (BODY)
/usr/bin/printf "%b" "$4" | /bin/mail -s "$3" "$2" -- -f $1

I know I could use the sendmail command directly (like this) but it would mean reworking a few scripts to take care of building the entire message including all headers.

Is there any way to specify additional SMTP headers using /bin/mail, in my own case I am trying to add the Importance: high header?

I'm running exim 4.63 as my MTA and CentOS 5.6 x64.

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up vote 6 down vote accepted

There are many different versions of mail (see Mail vs. mail what is the difference and the Heirloom project's write-up on the different versions of mail).

If you want to keep your sanity, I recommend avoiding any tool whose name is too close to mail. Mutt is a lean text mode mail client which is often available, very flexible, and behaves the same everywhere.

mutt -H - "$2" <<EOF
From: $1
To: $2
Subject: $3
Importance: high

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What's does the middle standalone - do? – Kev Feb 2 '12 at 1:34
@Kev The option -H takes an argument, a file to read the mail contents (headers plus body) from. I pass the argument -, meaning to read standard input (and not a file called -; this is a very common convention in the unix world). – Gilles Feb 2 '12 at 1:39
Thanks Gilles, works a treat. I never knew about the - convention before, will remember that in future. – Kev Feb 2 '12 at 1:45

No, there's no way. Header syntax is complicated and tools like mail were designed to limit user exposure to it. To use arbitrary headers you should submit the message using sendmail.

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Yes, use the -a flag, e.g. -a "Importance: high".

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-1 As per the linked server fault post, that doesn't work. – Kev Feb 1 '12 at 22:14
For Heirloom mailx 12.4 this option is -a filename (Attach the given file to the message). Not what I was looking for! – dsummersl Aug 28 '13 at 20:50
The -a option (for adding custom headers) is for bsd-mailx, not by heirloom-mailx. I do not see an equivalent option to add headers for heirloom-mailx. – DanB Aug 4 '14 at 11:19

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