It is not really advisable to be copy pasting from a Word Processing program into a Write email message.

Word processing packages have a load of options that are designed specifically to create more complicated Word documents. If you are creating a specific Word document then add it as an attachment, so that the recipient can open it and view it with the correct software, to read as it was intended. They can also detach it and save accordingly.

When pasted into something that uses HTML, it will copy paste a load of extraneous coding rubbish that bloats the email. It can cause all sorts of formatting issues to various degrees, some of which you have been experiencing. Send an email to yourself with something copied from a Word document. View the received email in Message Pane click on 'More' > 'view source' to see the extensive pages of coding that can be produced.

Assuming you are composing using HTML, it is best to use the Formatting Bar in the Write window to create emails as it has everything you would need to create even more complex emails. When you paste a short copied section, do you use:

'Edit' >  'Paste as Quotation'

or

'Edit' >  'Paste without formatting'

or do you set the Formatting Bar option: 'Preformat' then paste in the section then on a new line reset Formatting Bar option to 'Body Text' before continuing with content.

Just in case you are not aware; emails you send may not look the same on the recipients computer. Do they have the same fonts etc? Are they using the same OS ? Do they use Plain Text ? Have they set up conditions to override the display?

Everyone can set up their email program to display how they want it displayed, which may not be the same as you intended.

A friend of mine came round and saw an email they had sent to me. They were suprised to see that the email looked very different from what they thought they had sent; they had no idea that the recipient preferences and operating system could effect it.

The HTML editor in Thunderbird is something of a sore point. It has many oddities but there seems to be little will and almost no effort directed to fixing them. It has been described as "unmaintainable".

An alternative approach to your problem might be to paste without formatting. That can be found in the menu and IIRC, ctrl+shift+v will do it as well. The result is that the pasted text sheds its own formatting and so takes on what you're using in the Thunderbird email message.

Or you could paste away, ignoring the inconsistencies, and do a one-time fix at the end by using ctrl+a to select everything and then re-apply your chosen font and sizing. Well, it usually works, most of the time.  :-/

Some users go at it another way. Do all the assembling and editing in a "better" editor (usually Word, shudder!) then copy and paste into Thunderbird. Unfortunately, Word produces reams of redundant formatting definitions that are never used, unnecessarily inflating the size of your email messages. Microsoft are not especially concerned about compatibility, interoperability, industry standards or efficiency. Libre Office Writer, by comparison, produces smaller documents with neat tight markup.

Plus what Toad-Hall said!

Thank you both for taking the time to deal with my question. At least now I know not to be looking for such a tool as it doesn't exist.

I'll investigate your various suggestions.

I would attach a document if I wanted to write much, but in the cases I mentioned I simply wanted to quote a sentence from someone else's document. I'll look at the different ways of pasting, as that sounds promising.

Thanks.