primary / backup mail server

[Vu Pham]

Recently some incoming email to  my mail servers were delayed from eight 
hours to several days. I did some research and found the following things:

1. Some emails were from ISP-type mail servers and some of those mail 
servers were listed as spam servers. Those emails were getting bounced 
back and finally got to my mail server after their mail servers ' status 
were cleared from spam servers. ( I am surprise to see many business 
emails using ISP-type mailserver instead of having their own mail servers ).

2. Some emails were trying to contact the backup mail server instead of 
the primary one.

For the emails of the second case, why did they try the backup mail 
server ? Both the primary and backup are in the same network, same 
communication link from the ISP.

Does it mean the link was overloaded so the remote mail servers have to 
try the backup one ?

Vu