I have been working on this weird problem for two days now. I have a PHP application that creates a PDF file and then emails it as an attachment. In my Thunderbird client (v24.5.0) I have four accounts which are set up identically apart from the mail server, username, password, etc. In fact everything else is default. When I send the email to two of these accounts, it arrives as intended, with the PDF attachment. When I send the email to the other two accounts (the same ISP) there is no PDF - I get embedded binary instead. I closed down the client, sent the email to one of these latter addresses and used the ISP's webmail to see what was on the server - the PDF was there. I restarted Thunderbird and downloaded - no attachment! I then downloaded a different client (Sylpheed) and configured it for one of the offending addresses - the email arrived with the PDF intact. I had left the email on the server. I then forwarded the email with the attachment to myseld at the same address and closed down Sylpheed. Back in Thunderbird I received 2 emails - the original with no attachment, just binary embedded, and the same email forwarded via Sylpheed which does have the attachment. I downloaded TB Lite and get the same result - no PDF, just embedded binary. Can anyone explain to me this different behaviour with identical accounts in the same client (and how to fix it)?