I have received an email from Mr. A. In court Mr. A denies sending this email saying that I am making up with everything. How can I prove that this email was sent from Mr. A?
You can print a copy of the email, ideally with metadata showing, and testify that you received it and did not modify the email.
The lawyer in the case against Mr. A can ask Mr. A if that is his email address, and can ask other people who have received email from him if that is his email address.
Mr. A's computer and/or email provider can be subpoenaed. The computer can be forensically examined for evidence that the email was sent or deleted, and the email provider can be asked to provide evidence that the email was sent.
Your email provider can be subpoenaed and asked to provide evidence that the email was received.
In general, people who deny sending emails in court who are confronted with copies of those emails authenticated by the testimony of the person who allegedly received the email lose their cases and are not believed by the judge or the jury. This doesn't always happen, but it is by far the more common outcome.