I originally posted this about six years ago but it still holds true.
I have probably deleted a lot of valuable emails.
Why?
Not 
because they are in the spam bucket, but because they look like 
spam. Like most business owners, I get tens to hundreds of emails a 
day. (Of course, having over sixty  email accounts doesn't help). Many of 
them are spam; many are not. The spam catcher does a decent job as 
it rarely gives a false positive on emails. But I digress.
In order to filter out more spam, I scan the subject line and 
from line of each email. If the subject line looks spammy, out it 
goes. If I do not recognize the from, out it goes...and therein lies 
the problem.
I have had many emails sent by legitimate 
companies both as initial emails and emails in response to one I 
sent. but, the from name is unrecognizable or it is a name 
of someone in the company. Period. Out it goes.
One innocuous example was I put a hold on a book at our local library. Several days later I received an email with someone's name on the from line who I didn't recognize. I accidentally clicked on the message and it was from our library. I sent an email back to them explaining what happened and suggested they include the library name in the from line. Next time I put a book on hold, the email notice had the library name included on the from line.
Oh, if the email looks legitimate, I do check the from email address. Knowing this can be spoofed, when I read the content, I check the links, if any. Sometimes the links will indicate one thing but link to an unknown, spam URL.
