Re: UIDL

["Edward J. Sabol" <sabol@alderaan.gsfc.nasa.gov>]

Excerpts from [ding]: (18-Mar-99) UIDL by =?ISO-8859-1?Q?Fran=E7ois Pinard?=
> Hi, people. I often noticed things like:
>
>    X-UIDL: 3f52d1362f2c1d57ae074fac92f7c9a0
>
> Would someone be kind enough to explain what it is, and why it is there?

It's a non-standard header inserted by certain POP3 or POP4 servers as a
method of storing the information for the `UIDL' POP command. It's like a
Message-ID basically. Unfortunately, when the client downloads the e-mail
from one of these servers, the e-mail still contains this `X-UIDL:' header.
There are at least two known POP servers which do this. The most common of
these POP servers will use exactly 32 hexadecimal digits as the value for the
`X-UIDL:' header. An example of which is quoted above.

A lot of spam, especially from about a year ago or so, was being sent out
with `X-UIDL:' headers. I think the intention was to fool these POP servers
into displaying the spam e-mails at the top of the recipient's inbox or
something like that.

Assuming you don't receive your e-mail from a POP server which inserts an
`X-UIDL:' header (and doesn't strip this header from the e-mail when
downloading it to your client), then you can definitely treat any e-mail with
an `X-UIDL:' header as spam *unless* it was `Resent-To:' you. That's an
important part caveat. If someone who does get his e-mail from a POP server
which inserts these `X-UIDL:' headers resends an e-mail to you or to a
mailing list that you subscribe to, then that valid, non-spam e-mail will
have an `X-UIDL:' header.

Later,
Ed