Question on managing two accounts

[somebody]

Hello,

I recently downloaded zimbra to manage my emails from my yahoo and gmail account. Now both of those email accounts as follows:

myname@yahoo.com and myname@gmail.com (they both are the same except for @domain.com)

so the following experiment involves sending emails using zimbra desktop:

Sending email from yahoo account to the same account, is fine.
Sending email from gmail account to the same account, is ok but for some reason, my gmail sent folder is highlighted showing 1 new message that is in bold letter like when you get a new email in your inbox and the sent folder says: Sent (1) (note: this doesn't happen when sending email from yahoo mail to the same account. Even though yahoo account will show sent email but that sent mail is not highlighted)
Sending email from yahoo account to both of my accounts for some reason shows sent email in yahoo sent folder (without bold letters) and in gmail sent folder (with bold letters). Yahoo sent folder makes sense but having that email in gmail sent folder doesn't make sense since I used yahoo account to send out my email.
Sending email from yahoo account to ONLY gmail account shows the samething where my yahoo sent folder shows the sent email (normal) but gmail sent folder also shows that message (with bold letter).

Any ideas why? thanks