A Single EMAIL ID is repeatedly exposed to Brute Force

Dear Team, Today I have faced a strange issue. Only one email account in the domain name is not logging in via Webmail, Mozilla Thunderbird, or any other mail client. After long research, found it was a Brute Force Attack. Accessed CPHulk and saw so many entries for that particular email address. 1. I have white-listed my IP but still, I am not able to log in. 2. Cleared all reports and tried. Successful in only one attempt and again I was thrown out. Need your support in this.