[SFUserAccountManager sharedInstance].currentUser = ([SFUserAccountManager sharedInstance].allUserAccounts)[0];

for (Case caseObj :Trigger.new) {
        if(caseObj.Origin == 'Email') {
            EmailMessage toEmail = [SELECT ToAddress FROM EmailMessage WHERE Incoming = true LIMIT 1];
            String te = String.valueOf(toEmail);
            System.debug('Email To Address is ' + toEmail);
            caseObj.Customer_Support_Email__c = te;
        }

    }

For(Opportunity opp : trigger.new)
     { 
       If(!SetEmaiIDs.contains(opp.Email__c)) // checking if added email is already not present on some other account
       {
         opp.Account.Account_Email__c  = opp.Email__c; // This line is throwing error

       }

for (Case caseObj :Trigger.new) {
        if(caseObj.Origin == 'Email') {
            EmailMessage toEmail = [SELECT ToAddress FROM EmailMessage WHERE Incoming = true LIMIT 1];
            String te = String.valueOf(toEmail);
            System.debug('Email To Address is ' + toEmail);
            caseObj.Customer_Support_Email__c = te;
        }

    }