I have a weird problem going on.
Whenever I send mails from my company's email account on PostBox (similar to Thunderbird for those that don't know) it ends up in the spam folder on the receiving end (Gmail, Hotmail, Yahoo etc).
But when I log into our hosting company's web mail and send an email from there (same email address), it ends up in the Inbox like it should (and not in the spam-folder).
I did a test where the body of the two emails were the same.
Any idea why this might occur?
Send emails from each source to an email address and look at the headers ('show source' 'show original'). Evaluating the information there should tell you if the mail client is using the same server as the webmail client. If you use a Yahoo! or Gmail account, you can also see if the email passes the SPF and DKIM tests ('pass' or 'fail'). It's possible if two different servers are being used and there is an SPF record and DKIM record, that the record isn't set up for both servers.
One difference is where you're sending the mail from (different IP's and possibly countries (your company and your ISP)).
Test with plain text email.
Try and send the mail from another mail client (Thunderbird for example as you mention it) to see if the same issue persists.
Test using PostBox from another location.
These few tests should pin point where the issue is.
