Yes but it would be unwise and the employer would have a hard time proving that the discipline or termination was for just cause in light of the recent and continuing events.
The employer is supposed to provide a safe working environment and thus if you have concerns, should request that the employer provide the following:
1. training for all employees with respect to handling of mail, looking out for suspicious mail etc.
Posted placards, inter-office memos etc. with info from the USPS. OSHA etc.
2. employee equipment/wear such as gloves, masks etc. for mail handlers.
You have a valid concern and I share the same concern.
All mail sent to my office that deos not have a return address or a recognizable return address is not opened, bagged and thrown out. No wonder I'm getting more postcards and email lately.