associate default from addresses for accounts

associate default from addresses for accounts

I am using zoho webmail to send and receive emails from my pop3 accounts.

I have the associate default from addresses for the pop3 accounts set to the same address as the account.

For example if the account I am accessing via pop3 is aa@domain.com, I set the associate default from address to aa@domain.com

This seems to work OK most of the time but sometimes the people receiving my email will see in the reply to field of my email: aa@zoho.com.

This leads to people either replying to my zoho account or asking me which account to use. 

Have I done something wrong in my settings, is there a bug in zohomail that causes this or I am doing something wrong?