How to distribute email's delivery between 2 or more servers

We provide Email Marketing service through our online App. We have about 30 customers. And each one has it's own mailling list (5k to 20k emails each).

What we really want is to distribute email's delivery between 2 or more servers. I was wondering What kind of aproach/solutions MailChimp , Constant Contact uses to provide a great service ? use many servers ? many IPs ?

Our spam policy suspends ANY user/customer that gets 10% bounced .


We have 1 server at The Planet

Dual Xeon Quad Core, com 12 GB of RAM

Processor #1 Vendor: GenuineIntel
Processor #1 Name: Intel(R) Xeon(R) CPU           E5405  @ 2.00GHz
Processor #1 speed: 1995.080 MHz
Processor #1 cache size: 6144 KB

Processor #2 Vendor: GenuineIntel
Processor #2 Name: Intel(R) Xeon(R) CPU           E5405  @ 2.00GHz
Processor #2 speed: 1995.080 MHz
Processor #2 cache size: 6144 KB

Processor #3 Vendor: GenuineIntel
Processor #3 Name: Intel(R) Xeon(R) CPU           E5405  @ 2.00GHz
Processor #3 speed: 1995.080 MHz
Processor #3 cache size: 6144 KB

Processor #4 Vendor: GenuineIntel
Processor #4 Name: Intel(R) Xeon(R) CPU           E5405  @ 2.00GHz
Processor #4 speed: 1995.080 MHz
Processor #4 cache size: 6144 KB

Processor #5 Vendor: GenuineIntel
Processor #5 Name: Intel(R) Xeon(R) CPU           E5405  @ 2.00GHz
Processor #5 speed: 1995.080 MHz
Processor #5 cache size: 6144 KB

Processor #6 Vendor: GenuineIntel
Processor #6 Name: Intel(R) Xeon(R) CPU           E5405  @ 2.00GHz
Processor #6 speed: 1995.080 MHz
Processor #6 cache size: 6144 KB

Processor #7 Vendor: GenuineIntel
Processor #7 Name: Intel(R) Xeon(R) CPU           E5405  @ 2.00GHz
Processor #7 speed: 1995.080 MHz
Processor #7 cache size: 6144 KB

Processor #8 Vendor: GenuineIntel
Processor #8 Name: Intel(R) Xeon(R) CPU           E5405  @ 2.00GHz
Processor #8 speed: 1995.080 MHz
Processor #8 cache size: 6144 KB

When all our customers are delivering campaigns at the same time (it happens before commemorative dates) we are getting too many rejected emails due the higher delivery flow. We rotate our mail outgoing IPs using 45 IPs. And control/limit the flow to a host ( eg: @hotmail , @gmail) from the each IP.

Our approach is

  1. Our Application delivers 300 emails to @hotmail.com at 2pm
  2. App blocks deliveries to @hotmail.com and start delivering to diferent hosts until it raise the limit by host
  3. App rotates the Mail Server outgoing IP ( using 45 diferents IPs ) every 10 minutes

I 'm affraid , as the business grows and we get more customers , the DELIVERY FLOW gets higher and this approach FAILS by getting rejected by destination hosts .