Botnets are Spamming through web-mail services like gmail.com,hotmail.com,etc. by compromising email-accounts

I read a post from this forum about how online accounts are compromised.
Following are some:

1.Same password to register everywhere

2.Common password

3.Phishing 

4.Malware. Keyloggers

5.Secret question like (what's my name or my favourite color)

6.Malicious browser extensions,firefox user-scripts/plugins/extensions

These compromised accounts are mostly used by spammers/botnets to spam.

Could anyone kindly elaorate.

How do the individual bots of botnet not get caught for using the compromised accounts to send spam mails.Gmail,Hotmail are big fishes,how do these bots evade detection?

Thanks