Clarification about Local Spam by Sender report

[rfg76]

I'm receiving a report from our fortimail unit, and there's this table "Top Local Spam by Sender" 

sbs@.. and avisos@... are registered accounts on the FM unit, and they are used to send alerts and reports to others users, mainly to internal addresses, but also to some external.

Other's accounts (ie agm@...  or info@...) are not registered on our unit.

[ul]

does this mean that this accounts are actually sending spam to other users?

how can I know why the emails from legitimate accounts are detected as spam?

how can I stop not existent accounts from send emails?

what others actions should I take?[/ul]

Thanks in advance

Robert.