From MozillaZine Knowledge Base

Note that Thunderbird versions before 2.0.0.0 and SeaMonkey versions before 1.1.1 did not add the ".eml" suffix to the attachment (bug 220646). Also, an enhancement request (bug 230448) was filed in 2004 to change the default for forwarding e-mails from "As Attachment" to "Inline"; this bug was resolved as "expired" then, but with the increasing problems of ".eml" attachments being blocked by service providers, it might be worth revisiting that request. --Rsx11m 01:27, 5 September 2007 (UTC)

• The fix for bug 380354 will introduce a preference mail.forward_add_extension to toggle the ".eml" file-name extension for the message attachment, Thunderbird will also get a checkbox for this in the composition preferences. This information should be added to the article once TB 3.0 and SM 2.0 are released. --Rsx11m 02:24, 15 January 2008 (UTC)
• The forwarding default is changed to "inline" for Thunderbird 3.0 per bug 230448. --Rsx11m 14:18, 12 July 2008 (UTC)

The "Forward As" extension is also available from the developer's web site. Note that the context menus for Reply and Forward were available up to Netscape 4.x, and a respective request to get them back pending with bug 17796 since 1999. --Rsx11m 02:09, 12 September 2007 (UTC)

• The "Forward As" context menu to the Forward button will be available with SM 2.0 (again, add this information once released). --Rsx11m 02:24, 15 January 2008 (UTC)

When double-clicking on an ".eml" attachment, the attached message pops up in a separate window and appears like a self-contained message. There are a couple of issues though, tracked in bug 204350 --Rsx11m 04:18, 9 November 2007 (UTC)

The patch for bug 161775 will disable the third workaround to include an attached message in the message body of a reply with the next major releases, unless bug 384599 results in making this behavior configurable. --Rsx11m 02:24, 15 January 2008 (UTC)

• The patch to make this behavior configurable has now been approved and checked in as well. Thus, on TB 3.0 and SM 2.0, you have to set "mail.reply_quote_inline" to true now in order to use the reply workaround with attachments included. --Rsx11m 14:02, 30 May 2008 (UTC)