You are probably looking at this page because I sent you a link. I have been asked about the why's and wherefor's of email being rejected or being marked as Spam before, so I have written this short guide.
There are two things which can happen, wrongly identified Spam sources, and emails which really do look like spam.
If you are genuinely sending email which could not be considered spam, first thing is to let the recipient know their server is wrongly rejecting email. This is cure-the-disease, and corrections made to the remote server will help everyone. If you cannot get them to change, you could try mask-the-symptoms, by making your mail NOT look like spam.
Please note, we cannot make changes to the recipients system, only the recipients techies can do that, so you must start there.
There are some things to avoid, which will help the Servers which route your email understand that it is not Spam.
If the (remote) server is complaining about the fact that your email looks like spam, try changing it so that the email looks less like spam:
I hope this helps you get your email through, but if it does not, please ensure the server rejecting the email is corrected, rather than berate our server, which kindly took the time to tell you about the rejection.