Not sure if anyone has had this issue regarding the dropbox file link service in Thunderbird, but I have been receiving this bounce back message when trying to send out emails. It was suggested by tech support to see if dropbox had an entry that could be added to our current SPF record to remedy this issue. I contacted dropbox support through their chat service and they were completely oblivious to what I was I was asking for. I got nowhere with the conversation and was given scripted responses and directed to useless links and FAQs. Not sure how many people out there use Thunderbird's desktop application as their mail client but any suggestions would be appreciated:)
