Sieve mail filtering language is a historical application created in 2008.

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Example code from Wikipedia:

# Sieve filter

# Declare the extensions used by this script.
#
require ["fileinto", "reject"];

# Messages bigger than 100K will be rejected with an error message
#
if size :over 100K {
   reject "I'm sorry, I do not accept mail over 100kb in size. 
Please upload larger files to a server and send me a link.
Thanks.";
}

# Mails from a mailing list will be put into the folder "mailinglist" 
#
elsif address :is ["From", "To"] "mailinglist@blafasel.invalid" {
   fileinto "INBOX.mailinglist";
}

# Spam Rule: Message does not contain my address in To, CC or Bcc
# header, or subject is something with "money" or "Viagra".
#
elsif anyof (not address :all :contains ["To", "Cc", "Bcc"] "me@blafasel.invalid", 
header :matches "Subject" ["*money*","*Viagra*"]) {
      fileinto "INBOX.spam";
}

# Keep the rest.
# This is not necessary because there is a "implicit keep" Rule
#
else {
     keep;
}
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Last updated February 11th, 2019