Strange things happening with forwarded mails

[mad <mad@sharktooth.de>]

Hi!

I am forwarding emails from a big email provider to my private email
server (postfix, dovecot).

At the email provider the first line of the email headers is:

Received: from X.X.X.X...

When the mail is finally put into my mailbox at my own email server this
is no longer the first line but there are a lot of headers before the
Received line but the strange thing is, that the line from above now has
additional characters at the beginning:

Received: from X.X.X.X...

This leads to problems with Thunderbird (Icedove) displaying the mail
(no Subject, no From). Interestingly, this only happens with mail
already tagged as spam by the email provider.

Does anybody know why this is happening? What the problem is?

Thanks in advance.

mad