A workflow is made up of a sequence of events such as workflow initiation, task creation, and task completion. When you add a workflow, you specify a task list and a history list that workflow instances of that workflow association will use to track the events for the workflow. The task list contains items that users interact with and enter data into while interacting with a workflow. The workflow history contains key information about each event including date, status, participant, and description.

When you add a workflow association, you assign a task list and history list to use with the selected workflow. You can use the default task list, select another existing task list, or create a new one.

Workflow tasks—as any other tasks—are displayed in task lists. When the user navigates to a task list, the tasks it contains are displayed based on the selected View and the access control list (ACL) rights of the user. A workflow can have different types of tasks, but all the tasks from a given workflow association are contained on the same task list. A given task list, however, can contain the tasks for multiple workflow templates and associations.

The workflow History list is an hidden list that contains key information about each workflow event including date, status, participant, and description;the Visual studio 2010 wizard will create it for you.By default the workflow history is not visible 60 days after a workflow completes or is cancelled;you will find more information about the Workflow Auto Cleanup Job here

When we create a workflow using Visual Studio, Visual Studio does this association for us. That means normally we do not have to bother much about this association. While adding the workflow itself, Visual studio will create History list and Task list association of the workflow. But for any reason this if association goes away, e.g. you mistakenly change or delete this from designer file of workflow…etc. You will face error during deployment.

There are two solutions to this problem:

1. In the Visual studio solution explorer, right click on the Workflow --> Properties. Set the Auto Associate attribute to false. Try redeploying your workflow, it should work now. :) As in this case , we have not auto associated the workflow with any history list or task list, user will have to manually go and create the association. 

    2.   If you still want the workflow to AutoAssociate, to reduce the steps for  manually associating it, you need to follow these steps:

       - In the Visual studio solution explorer, right click on the Workflow àProperties. Click on  the ellipsis button (…) at the right most to the History List Attribute.

       - On the Next Screen Specify the Name you wish for your workflow. The workflow will appear with this name on the site.

        -  On the Next screen, check the checkbox which says "Yes, associate this workflow with the following libraries and lists. if this step is ommitted, you must manually associate the workflow after it is created." The history list  dropdown and task list dropdown will contain all the lists those are available for association. Keep the default ones or choose the one you wish to.  Click on finish.

        - Now you will see the History List and Task list are associated with the workflow.

        - Try redeploying your workflow now. It should not give any deployment error. :) 

Depending upon your requirement i.e. whether you need auto association of workflow with history and task list, you need to choose the solution.

Other References :

https://msdn.microsoft.com/en-us/library/ms563325.aspx

https://msdn.microsoft.com/en-us/library/bb417400.aspx

1. Login to Exchange server. Go to Exchange Management Console.  Expand Server Configuration option. Click on Hub Transport.
2. You need to create your own relay connector. How to create and configure the relay connectors, you will find it here .
3. After your relay connectors and receivers are configured, you are all set to configure your servers which need to send mails. Here I have configured the receiver named sharepoint. Right Click on newly created receiver connector. sharepoint--> Properties--> Network tab. In the Receive mail from remote servers that have following IP’s section click on Add tab and enter the IP of the machine you want to enable for sending emails.

 
After you configure the exchange server, you may need to configure the mailboxes for AD users on exchange. Here is the way to do it.

 1. Login to Exchange server. Open Exchange management console. Click on Recipient Configuration --> Mailbox. On the Right top corner, in the Actions dashboard, click on New Mailbox.

2. Click on User Mailbox in the next dialog box.

 3. If you have an existing user in the AD then select Existing user otherwise create a new user. Here we already have users in the AD.

4. Click on Add. Next screen will show you the list of all users from the AD, whose mailboxes are not configured. Select a user whose mailbox you want to configure and click on Ok.

5. Click on Next.  On the next screen, specify the alias for the user. If you try to configure the multiple mailboxes at the same time, exchange server will take the alias by default. You will see this option disabled. Click on Next.

6. Click on New. It will create a new mail configuration for the user.

7. Exchange will configure the mailbox for the user. Click on Finish.  Right click on Mailbox and say Refresh. If will show you newly created mailbox.

 Now as your exchange server is configured, you need to configure the outgoing email setting in the Sharepoint: 

1. Go to Central Administration. Click on System Settings à Configure Outgoing e-mail settings.
2. In Outbound SMTP Server Textbox, enter the name of the exchange server.
3. Enter valid email addresses  in From address and Reply to address. If you wish to specify these options through your code then you can leave this blank.

    4. Both Exchange server and Sharepoint are configured. You are all set to test the email functionality either through your code or using telnet or Sharepoint alerts or adding a user to any Sharepoint group which automatically notify user by email. 

Happy Mailing !!!