Over the last few months I've noticed many of our clients' email servers rejecting mail sent by me via Thunderbird. When I send mail or reply to mail via iphone, ipad, or the web client for gmail (our mail is hosted by gmail for business) there isn't a rejection (typically a 554 denied, rejected by server; or 554 service unavailable, client host blocked using barracuda reputation. I had an html signature with an attached logo for a while, which is when this seems to have started but may just be a coincidence. I've since removed the logo and html signature but the errors continue...

What can I do? I don't know of a better way to handle email on my Mac.