Why there is no Rest API for getting the 'accountID' by email address of the customer?

[Ima Najeeb]

I don't want to try it with username as usernames can be same.

Also, In my problem, I am initially having customer's name and email address only, and then want to check if it already exists in my JIRA service desk or not. 

Please tell me how to do it so that I can get accountID of existing customer and then can create a new issue on his part?

[Achala Dias]

you can use the query to filter any specific user.

https://developer.atlassian.com/cloud/jira/service-desk/rest/api-group-servicedesk/#api-rest-servicedeskapi-servicedesk-servicedeskid-customer-get

example: 

https://<your-org>.atlassian.net/rest/servicedeskapi/servicedesk/1/customer?query=testuser@gmail.com

this will return all records if there any user with an email address "testuser@gmail.com"