I have an LWC Component which have two buttons one is for new mail and another one is for reply mail there is no issue in sending new mail, but for reply mail button when user click it, need to send reply mail for last sended mail of lead record, I will refer many in internet but i will not get it, need your guidance in it

I have created a class with without sharing keyword and I have provided access of this class to guest user profile. This class is being called from a LWC component which is hosted on public support site in salesforce. Inside that class I am accessing a Metadata field to get key. Later I am calling some rest endpoint using this key.