You can attach any file to a message while it is being composed by drag-and-drop onto the addressing box above the message text. This applies to other email messages too.

But, why not use "forward" to pass on the old message? Select it, click "Forward" and it'll open in an edit window, with the old message either displayed inline or as an attachment, according to your preferences set under Tools|Options|Composition|General.

need the headers. thx for help.

unfortunately that didn't work after all. any other ideas. thanks.

In what way did it not work???

Most agencies I send spam to want the whole message verbatim and sending it as an attachment almost always works best. (The awkward ones require you to copy and paste into a web form, or similar.) I use a "Forward As" add-on so I can choose between attached or inline on the fly.

You can view headers, and indeed the whole message source, by use of ctrl+u.

Most of the agencies who manage spam or phishing messages don't like the message text being included inline, or indeed copy-and-pasted, because it's hard to avoid it being re-flowed and thereby breaking up long lines. This damages the message's structure and makes it impossible to automatically parse the headers.

the new message wouldn't accept the "drop".

thanks for info. i'm already aware of all that. i just have this one little issue in thunderbird. do appreciate your help.

You're dropping onto the address box? Have you tried this with any ordinary file from your file system? Is it drag and drop that fails, or is it just when the target is an email message?

Another approach: open the offending message, save it as an .eml file.

Attach the .eml file to your new message using any of the available methods, such as the File|Attach procedure.

why not use "forward" to pass on the old message? – need the headers

Go to the message you want to forward, then select Message → Forward As → Attachment. This will attach the message as is, i.e., without making it part of your own message and thus recoding its content.

Did you try this, and if yes, which problem did you encounter?

i thought i hit the addresss box but apparently not. thanks for help

Just as a follow-up: There is actually a difference between forwarding as attachment and using drag-and-drop for a message. With "proper" forwarding, the attachment's name will be the subject of the message with ".eml" added (there is a preference setting to switch the latter off). However, drag-and-drop will just name it "Attached Message" regardless of the message's subject.