trigger LaatsteCallMobileMiles on Task

/*
 * @see https://success.salesforce.com/answers?id=90630000000hdCVAAY
 */
trigger LaatsteCallMobileMiles on Task (after insert) {

    // Retrieve the email addresses of CreatedBy users for tasks currently
    // being processed by the trigger

    List<Task> newTasks = [
        SELECT Id, WhatId, Subject, CreatedDate, CreatedBy.Email
        FROM Task
        WHERE Id IN :Trigger.new
    ];

    // Initialize a map of accounts to update, based on the Related To
    // (WhatId) field on the task. The map will be keyed on the Account ID,
    // to make sure the same account is not included twice in a single DML
    // operation.

    Map<Id, Account> affectedAccountsById = new Map<Id, Account>();

    // Go through the new tasks and check to see whether the account shoudl
    // be updated, based on the email address and subject criteria

    for (Task eachTask : newTasks) {
        if (eachTask.WhatId != null) {
            if (eachTask.CreatedBy.Email.contains('@mobilemiles.nl')
                && eachTask.Subject.contains('Made call to')) {

                // Construct the Account record to update

                Account affectedAccount = new Account(
                    Id = eachTask.WhatId,
                    Laatste_activiteit_Mobile_Miles__c = eachTask.CreatedDate);

                // Put the affected account in the map

                affectedAccountsById.put(affectedAccount.Id, affectedAccount);
            }
        }   // if (eachTask.WhatId != null)
    }   // for each Task in newTasks

    // Update any affected accounts

    if (!affectedAccountsById.isEmpty())
        update affectedAccountsById.values();
}