I agree with you on the fact its hard to accept that only ONE religion is right, and I too seem to be of the mindset that a religion or church isn't necessary to me to be a good Christian.
On the other hand, many people find great peace of mind and soul by being part of a congregation, a religion. They feel it is more comforting to be surrounded by like minded individuals who share in their joys and hopes. The "fellow feeling" of a church, sharing each others joys and sorrows of life, is very important to many people, and this is part of why they join or belong to a religion.
Instead of thinking that friends who are members of a church are "pushing off" their religion on others, think of this. Most likely, they are very happy and secure in their beliefs, and they want the same for you. They want you to feel the same peace of mind in this life and be ready to join them in the next life. They are not sending you emails because they wish you bad things, they are trying to "save" you, help you, invite you to join in their group.
I have friends in many different religions...mostly Christian, but many denominations. I accept their good wishes, their literature, and some of their invitations to events. I understand that they are being sincere in their wishes. I ask them questions on what they find their solace and peace in, and what they believe.
Since you have gone to many churches, I am guessing that you, too, are searching for something. The people you have met are just trying to help you find it.
As far as emails, the great thing about them is you can delete them and have no obligation to even respond to "chain" letters, etc. If it really bothers you, change your email address, and only give it out to a small group of people you want to hear from. Personally,I don't send on religious or political email stuff to people at all, unless they specifically ask me about something and I do some research and am sending that.
Just try to be gracious in your behavior. There is so much to learn from the different religions. If you want to, read those emails. If not, delete them. Its your choice.