Defamation case due to foul language in mail

A few days back I mistakenly bought an investment product from an online portfolio management service. Because of their misleading interface I got an impression that money will be deducted monthly. Besides they took bank mandate for a duration of one year which further reinforced that the money will be deducted monthly. But they deducted the money at once. When requested for a refund they refused saying that there is no provision of refund. I further requested them to deduct the money for one month and refund the rest, which they again refused. Infuriated, to one of the newsletters they send to their subscribers via email, I replied with one sentence containing word "Thief" and "bastard" in it. They have served me with a notice for defamation case. Meanwhile I had filed the complaint against them to SEBI. Initially they refused saying that they never promised monthly fee and their agreement does not allow refund. But after I submitted to SEBI their interface which made an impression of monthly fee structure, they refunded the money (after making deduction for one month). Do they have valid ground for defamation case and how should I apply to their legal notice?