1. How can I search for prior trademarks using USPTO - TESS database?

Here is the link to search for trademarks,

2. What do you mean by the following?
a. There was no live IMESSENGER trademark registered. 

Our application number is, 88019343, which was filed after researching for information. 

As you can see below, there are five other earlier applications with two showing registration numbers (Reg. Number). Out of those five, two of those were approved at the end and became official trademarks. 

The last column shows, "Live/Dead", which implies whether the trademarks (Registered and filed) are still live or dead. As you can see, except for the one that we filed, all the others were dead.

Please see below -
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b. There was also precedence for acceptance of the mark, IMESSENGER, not once but twice by USPTO in the years 2000 and 007.

As you can see above, there were two registered IMESSENGER trademarks listed in the database with registration numbers, 2356273 and 3348418.

c. Moreover, these were the first set of trademarks approved in the Messenger space.

What do we mean by this statement? 
Please see below are screenshots of the two accepted trademarks for the mark, "IMESSENGER", that are not live anymore. If we were to do further research in this space, you would see that this trademark was the first one in this space of messaging technologies.

d. Can someone claim that our app will not come under the Class 38 (Telecommunication services)?

As can be seen, both the IMESSENGER approved trademarks were also approved for Class 38. In addition, the office action letters that we received cited IMESSAGE, INSTANT MESSENGER, AOL MESSENGER applications, which all were classified under this class.

e. Why did we filed for Class 38?
We filed for class 38 as we had this messaging app in use for a while prior to filing and it was categorized under telecommunications in the App store. Further, prior art for similar messaging apps not confined to the above-mentioned ones, but also apps such as, Hangout and more were also classified under Class 38 too.

f. Will we be adding classes to this examination process?
We are considering to add additional classes as our product offerings align with some of the other products out there in the marketplace. Fortunately, prior art in this space would help us to determine all the classes that can be added to this application.

g. Are we claiming this product to be unique?
We never claimed anywhere that ours is unique, and also made it clear about our dependence on a third-party API for this application. At the same time, considering the available resources that we had, we developed this application aligning with our objectives to be self-reliant. We certainly understand that this is not complete self-reliance, but we are always striving and looking out to improve our offerings while relying lesser and lesser on third-party offerings. 

However, we would also like to make it clear that we certainly understand the importance of a collaborative and cooperative world where it is better not to be self-reliant by being totally self-sufficient in order to improve camaraderie and to create a sustaining and thriving society.