an email has two parts. a display name and an email address. The client can use whatever brackets it likes to display internally, but the form Display Name <email@domain.com> is what is required in the mail itself.

That these addresses are showing wrapped in inverted commas shows Thunderbird is in fact aware they are invalid, but it will not prevent you from using them, it will also add them the the address book if you try and use them.

I am guessing, but it would appear reasonable to me to guess Eudora used a slightly different system of brackets which has imported.

Ugh; I didn't say enough. The form "(realname / other comment) e-mailAddress" is in imported messages. That form was preferred and often is supported for outgoing e-mail although no longer preferred, but, it seems, Thunderbird does not know it.

show me where is says that please http://tools.ietf.org/html/rfc5321

If it is gone, I am not surprized—but some software yet supports it. If Thunderbird supports importing from Eudora, it would be seemly to support such older ways of decorating e-mail addresses. And I have older messages that I managed to "import" into Eudora that were sent by Fido.

You can use "email@address (real name)" as an alternative to "real name <email@address>"...

But my problem was that in importing from Eudora Thunderbird mangled such address entries.

No, you were talking about addresses in the form "(real name) mail@address", which is not the same as "mail@address (real name)"... As far as I know the form you mentioned is invalid, while the second form is valid.

Curious that after all this time, I now for the first time come across a practical application of that, in an impractical setting (we hav given up Eudora). But if I had programmed the software that let the invalid through, it would hav been just as permissiv as I hav found it.

http://users.starpower.net/ksimler/eudora/nickname.html edited in 1996 Tells about Eudora nickname/addressbook structure at the early stages