Excessive postage

Handwritten or poorly typed addresses

Unexpected or from someone unfamiliar to you. 

Incorrect titles

Title, but no name

Addressed to someone no longer with your organization or are otherwise outdated.

Misspellings of common words

Oily stains, discolorations or strange odor

No return address, or one that can't be verified as legitimate. 

Excessive weight

Unusual weight, given the size

Lopsided or uneven envelope

Protruding wires or aluminum foil

Excessive security material such as masking tape, string, etc.

Visual distractions

Ticking sound

Marked with restrictive endorsements, such as "Personal" or "Confidential"

Shows a city or state in the postmark that does not match the return address