Emails are flagged as spam at the incoming mail server or in the spam filters of the recipient address.
If all your outgoing email is being marked as spam this may be due to:
1) your domain having incurred a poor reputation and having been blacklisted at external blacklists (URIBL or RHSBL services) as a result of previous activity;
2) formatting of your emails;
3) inclusion in the body of your emails of a blacklisted URL (i.e. your domain has no reputation but in your email you are quoting a URL containing a blacklisted domain);
You can use the tool available at the following site, entering your domain name, in order to view any blacklists that list the domain and can request de-listing following each blacklist's respective procedure (links to the blacklists are provided): http://multirbl.valli.org
Following removal of any blacklisting of your domain, it will be necessary to ensure that all future outgoing mail is formatted and sent in such a way that blacklisting does not occur. There is a great volume of content online regarding how to avoid your emails being marked as spam (regarding format, sending of unsolicited emails etc.) that will helpful in this respect.
For any detailed advice on why emails are being flagged as spam please consult the administrators of the incoming mail servers, as it is the incoming mail servers that categorize mail as spam based on the specific criteria their administrators have in place.