No. You aren't sending too much email.
You're probably sending too little.
That's what I've experienced (and it's true of direct mail as well as email). It's also reported recently by Frogloop, at How many fundraising emails should you send a year?
... more good emails = more revenue.
That is completely and unambiguously true. More contact with donors means more revenue. It also means better donor retention.
I have yet to see any evidence to the contrary.
The belief that contacting donors a lot can cause them to leave in disgust? It's a myth.
Want more details? Listen to the recent Fundraising Is Beautiful podcast on this very topic: How many appeals should you send?