It's probably because it doesn't recognize or it has an abusive/wrong filter related to sender-client header.

Mine has:
X-Mailer: Modest 3.0

It's also possible that, if you use the PC as a gateway and the N900 as a client on the internal LAN that you have the issue of IPs.

The messages may get spam if the server sees that your declared IP doesn't match the actual IP. My phone, e.g., has 192.168.16.100 (16 is the WIFI NAT sublan) and the PC is <insert routable IP here>. The server sees you are "lying" about your IP and marks it as suspicious.

Just a few thoughts.