Here you will find information on suspect mail, this document offers guidance from Royal Mail.
Here is a guidance document from the University Security team.
The Health and Safety Executive also identifies various precautions for general mail handling at the following link http://www.hse.gov.uk/biosafety/diseases/anthrax.htm
Some indicators of a suspicious package can include;
If you are unsure of a piece of mail, please contact Security on 0117 9287848 for further advice and assitance.
The Centre for the Protection of National Infrastructure (CPNI) offers some good advice at the following link https://www.cpni.gov.uk/screening-mail-and-courier-deliveries