A Bash script to "validate"/check the syntax of e-mail addresses

mail_syntax [--echo=[in]valid|both] [--sloppy] {mailaddress}
mail_syntax [--echo=[in]valid|both] [--sloppy] -f {file}

In order to validate an e-mail address it is recommended to simply match it against a regular expression. mail_syntax essentially does the same but its name is easier to remember than the full regular expression. Moreover, it tries to be a bit more versatile.

{mailaddress} should be either a plain e-mail address or the address part must be enclosed in <> brackets. --sloppy will make the script try to find a valid address no matter what. For example, the following strings are considered valid addresses:

foo@bar.not
Foo Bar <foo@bar.not>
foo@bar.not (Foo Bar)
|foo@bar.not$                    ... though only with --sloppy

If mail_syntax finds {mailaddress} to be invalid it returns a non-zero exit code (FALSE). What is valid and what is not is defined within the script. It's more restricted than what the RFC 2822 allows.

  --echo=invalid ... write invalid address to stdout
  --echo=valid   ... write the extracted valid address to stdout
  --echo=both    ... write the address in any case

mail_syntax foo@bar.not
mail_syntax --echo=valid "Andreas <foo@bar.not>"
mail_syntax --echo=invalid -f somepath/addresses-to-check

Example of usage within a .procmailrc recipe

:0
* ^From: *\/[^ ].*
* !? mail_syntax "$MATCH"
{ LOG="Sender's address appears to be syntactically invalid"
  :0
  invalid-mails
}

1. The script is slow (compared to a single match against a regex).
2. Valid maximum length of addresses is not checked.
3. Local addresses are considered invalid (e.g. foo@local).
4. IP addresses like in foobar@[192.168.2.1] are considered invalid.
5. Quoted local parts like in "foo"@bar.not are considered invalid.
6. --sloppy is indeed what the name suggests.
7. Überfoo@foobar.not is regarded valid in --sloppy mode.¹⁾
8. Various locale(5) settings have not been tested.

• Use Bash regular expressions instead of sed.

# Check if address is empty or just spaces
[[ ${ADDRESS// /} ]] || { addrinvalid ; return ; }
 
# Check that there is at least an @ sign and a . (dot)
[[ $ADDRESS = ${ADDRESS/@/} ]] && { addrinvalid ; return ; }
[[ $ADDRESS = ${ADDRESS/./} ]] && { addrinvalid ; return ; }
 
# The main check is done with _sed_ and $REGEX (see script)
echo " $ADDRESS " \
| sed -ne "
   # remove quoted text
   /\".*@[^\"]*/s/\".*[^\\]\"//g
   # retrieve address without <...> and optional ending (...)
   s/^ \($REGEX\) *\(([$VC @.]\+)\)\? *$/\1/p
   # retrieve address within <...>
   s/^[^<]*<\($REGEX\)> *$/\1/p
"

v0.31 2011-04-25 removed futile test for vertical whitespace + control chars
v0.30 2011-02-27 replaced [ with [[, published here

I have tested this on Debian 4 + 5 only. Use at your own risk.

This script is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License; either version 2 of the License, or (at your option) any later version.

The tar.gz includes the script, a test script and a few lists of valid and invalid address examples for individual tests.

mail_syntax_0.31.tar.gz