Here's an interesting question that I was asked earlier today; I can't offer a definitive answer, but these are my thoughts. If you have any contradictory or complimentary comments, please comment or let me know.
"Can RTF/HTML Mail be as safe as plain text with regard to viruses/malware etc?"
Theoretically, I think plain text will always be safer since there’s less work for the server to do, and there’s no encoding of the content other than the real basics of wrapping up the envelope of the message (eg taking the various to/from/subject fields, encapsulating the blurb of body text, and turning it into an SMTP-formatted message).
Where things could get interesting is that plain text still allows for encoding of attachments (using, say, MIME or UUENCODE), which could still be infected or badly formed – so the risk level of attachments is technically the same (although in an RTF or HTML mail, the attachment can be inline with the text, which might mean the user is more likely to be lured into opening it, if it's malicious).
There may be some risks from a server perspective in handling HTML mail which mean that a badly formed message might be used to stage a denial of service on the server itself. I heard tell of a case a few years ago when a national newsletter was sent out with a badly formed HTML section, and when the Exchange server was processing the mail to receive it, the store process crashed (bringing Exchange to its knees in an instant).
The downsides with that scenario were:
- The message was still in the inbound queue, so when the store came back online, it started processing the message again and <boom>
- This newsletter was sent to thousands of people, meaning that any company that had at least one person receiving that mail, had some instant server-down until they identified the offending message and fished it out of the queue.
This bug in Exchange was identified & fixed, but there’s always the theoretical possibility that since the formatting of an HTML message is more complex, there could be glitches in handling the message (in any email system).
Plain text mail is ugly and so lowest-common-denominator, it’d be telling everyone to save their documents as .TXT rather than .DOC or .PDF.
RTF mail works OK internally, but doesn’t always traverse gateways between Exchange systems, and isn’t supported by anything other than Outlook (ie mailing a user in Domino, they won’t see the rich text).
HTML mail may be slightly larger (ie to do the same content as you’d do with RTF takes more encoding and it’s sometimes a bit bigger as a result), but it’s much more compatible with other clients & servers, offers much better control of layout and traverses other email systems more smoothly.
I'd say HTML mail is the obvious way to go. Anyone disagree?