Hello Tpw. Email companies classify messages on the basis of spam by looking at some common characteristics like content, subject, header, sender's email address, IP address, etc. You can try writing a subject as something related to your message so that it will also let the user know what the message concerns. Include fewer hyperlinks if possible and try not to write in all caps.
If that doesn't work still then it is probable that the Email provider has blacklisted your IP address or your ISP provider as a spammer and therefore your mails reach the spam folder.
In that case, there is a little you can do that contact your ISP. Hope that helps.
-- Goku