Is it possible for using imap to modify the message of email?

Question

is it possible to add a pop-up message on the received email?

I have searched IMAP but it can only fetch the email, view the email content. and I have considered milter, but it only uses for filtering. My idea is when there is a URL in the context of email, I could show a warning message to tell the users that it is a phishing URL or not. I would like to make the warning message is available for no matter what email client I am using.

Answer 1

Since you already mention milter in your question: a milter would be capable to modify the message bodies and can solve your problem.

The xxfi_body callback of libmilter allows you to obtain the message body. When the xxfi_eom callback is called, you can use smfi_replacebody to replace the body of the message.
This would allow you to search for URLs in the bodies and append a warning message to them.

Notes:

• You need to add the SMFIF_CHGBODY flag to the value of xxfi_flags when calling smfi_register, otherwise calling smfi_replacebody will fail
• Changing the message body may have a significant performance impact
• When configuring your MTA, the order of filters matters: later filters get the new body contents, when a previous filter replaces the body
• As noted in the comments: when you modify the message contents, things like PGP, S-MIME, etc. might break
• You might need to interpret the Content-Type of the message (and it's mime parts) so that you can distinguish between text/plain and text/html messages and append the warning message at the proper position

For more information on how to write a milter application, you can find the current HTML docs in the sendmail source under the path libmilter/docs/index.html. There is also an online version (may not be the latest one).