One possibility is that someone else on your service provider’s shared server has been blacklisted by IP address, which affects all users on that shared server.

In those situations, I ask my service provider if that is the case. If it is, I ask them for help getting off blacklists.

Getting results may still be difficult.

Besides possibly being IP blacklisted, there are some things you can do to avoid triggering spam filters. Avoid messages with remotely-hosted images, including signatures, send messages in plain text, and perhaps include a line such as "please whitelist this sender in your webmail settings so it's not sent to Spam".