Q: How can I ensure that attachments sent to my mailing list go through?

By Jacob Haller
Senior Support Engineer, L-Soft

LISTSERV has some sophisticated settings that provide fine control over what sort of attachments are allowed on a particular mailing list. However, a lot of the time list owners want something very simple. They just want to allow attachments of all kinds to go through unscathed. How can you make sure that happens?

The short answer is that while most attachments would be allowed with the default configuration, if you want to be certain that all attachments will be allowed, then make sure that your mailing list is configured with:

You may ask why these settings are necessary, which brings me to the longer version of the answer.

In the early days of Microsoft Outlook, Microsoft developed a proprietary way of formatting text that involved storing the formatting data in a winmail.dat attachment (technically, an attachment with the MIME type "application/ms-tnef"). They were annoying to people who didn't use Outlook, and LISTSERV removed them by default.

Outlook/Exchange will sometimes now include other sorts of files in winmail.dat files so removing the attachments may not be desirable. Misc-Options= KEEP_EXCHANGE_DATA tells LISTSERV to retain them.

The origin of Attachments= All goes even further back in time. Email was originally designed to transmit plain text messages, and various schemes were designed to allow the transmission of files via email without violating the email standards. Generally, they involved various ways of encoding the files as text. Your email program would then automatically detect that an encoded file was present and decode it for you.

Eventually, the scheme that won out was one named MIME. With MIME, the sending and encoding email program specifies what kind of file is in the attachment (text, HTML, jpeg, Word document, etc). Then the file itself is attached, usually encoded using the base64 encoding scheme.

LISTSERV normally allows all MIME attachments to be distributed. However, occasionally you'll see a situation where an email program sends an attachment using an obsolete encoding scheme called uuencode. Unlike MIME attachments, uuencoded attachments don't provide any information about what kind of file is in the attachment, so LISTSERV normally strips uuencoded attachments out – unless you use Attachments= All, in which case this type of attachment is also allowed to remain.

Additional information about the Attachments setting can be found at:
http://www.lsoft.com/manuals/16.0/listkeyw.html#kAttachments

Information about Misc-Options= KEEP_EXCHANGE_DATA can be found at:
http://www.lsoft.com/manuals/16.0/listkeyw.html#skKeepExchangeData

Site administrators who wish to make KEEP_EXCHANGE_DATA the default can do so via the DEFAULT_MISC_OPTIONS site configuration setting.

More information about this setting can be found at:
http://www.lsoft.com/manuals/16.0/sitevars.html#cDefaultmiscoptions

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