MIME Removal Services

• Introduction
• MIME Issues for Mailing Lists
• What is Filtered
• Notices to List Owners

Introduction

Many e-mail programs make it easy to send a computer file as an attachment when sending an e-mail message or to send a text message formatted using HTML. But sending non-plain text messages can be problematical, especially when an e-mail list of diverse subscribers is involved. For this reason, by default, messages sent to a UNH mailing list that are in MIME format are converted automatically (where possible) to plain text.

A better method for distributing large or complex documents is to publish them as a web page and send a reference to the page via the list. Since this alternative exists, we ask all list owners and subscribers to refrain from sending e-mail attachments via the mailing list system if at all possible. However it is recognized that for small specialized lists the ability to exchange files as attachments is key. In these cases the list owner may specifically request that MIME filtering be disabled. Note that this will not disable the automatic virus filtering which is performed on all messages sent through the UNH e-mail systems.

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MIME Issues for Mailing Lists

Here are some of the reasoning behind offering this service:

• To prevent the distribution of e-mail-borne viruses.

• To prevent unusable e-mail messages from being sent to a significant number subscribers who do not have the specific commercial software that would be required to read it. Some lists have a wide diversity of subscribers representing a heterogeneous mix of computer systems and software. As a courtesy, only messages that can be easily read by all subscribers should be sent to the list.

• To prevent denial of e-mail service to subscribers who have limited mailbox space quotas. Non-text attachments are often very large, requiring megabytes of storage, even for content that is the equivalent of just a few short paragraphs. One or two such attachments can fill a subscriber's mailbox, preventing that person from receiving any more e-mail until the offending messages have been deleted.

• To prevent degradation of the University's network performance to the detriment of other computer users. Large attachments sent to lists with large numbers of subscribers can cause a significant strain on the local e-mail systems and networks that host the list management system.

• To prevent the imposition of excessive time and/or cost on individual subscribers. Large attachments can cause lengthy downloads over slow modem connections which can result in significant additional time and expense for some subscribers.

• To prevent the interruption of list services to other list clients on our server. Distribution of a large posting can potentially fill the local server's e-mail queue, preventing it from processing any other requests until delivery of the posting has been completed. Because of potential delivery problems caused by other systems refusing to accept the resulting large messages, this condition could tie up the list server system for several hours, or even days if it happens while the University is closed.

Of course you don't have to just take our word for it.

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What is Filtered

All mail sent to the list's posting address, as well as the list's *-request address which is used for human-to-human contact with the list owner, is filtered using the open source demime mail filter program. The list's owner-* is not filtered since this address is used for reporting error messages, which are often sent in multi-part format.

Filtering is performed with one specific purpose in mind: to convert as needed all mail messages into plain text which can be viewed using any e-mail client. Filtering is automatically performed on messages sent in MIME format. This includes messages with non-text attachments (i.e. graphics, programs, scripts, etc.) as well as text sent in HTML or Microsoft's rich text format. (These formats allow the use different font sizes and styles, colored or patterned backgrounds, and embedded hyperlinks.) For messages that have alternative parts, demime will select the plain text version if provided. Failing that, it renders rich text and HTML alternatives to plain text. It also renders non-alternative parts when it knows how, and elides them when it does not.

Finally any recognized advertising boilerplate, which is often appended by free mail services such as Yahoo and Hotmail is removed. This is done to help reduce the size of the archives maintained for some lists.

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Notices to List Owners

• 15-SEP-2004: Service Inauguration Notice

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