Navish Jain MS Dynamics CRM Consultant

For Better Understanding I divided this article in multiple part
(i)      Create Email Activity
(ii)     Create an e-mail attachment
(iii)     Retrieve an attachment
(iv)     Update attachment
(v)      Retrieve all attachments associated with the email activity
(vi)     Delete Attachment

Namespace need to include 
using Microsoft.Xrm.Sdk;

using Microsoft.Xrm.Sdk.Query;
using Microsoft.Xrm.Sdk.Discovery;
using Microsoft.Xrm.Sdk.Messages;
using Microsoft.Xrm.Sdk.Client;
using Microsoft.Crm.Sdk.Messages;


Create Email Activity
Email email = new Email

{
    Subject = "This is an example email",
    ActivityId = Guid.NewGuid()
};
Guid _emailId = _serviceProxy.Create(email);


Create an e-mail attachment
ActivityMimeAttachment _sampleAttachment = new ActivityMimeAttachment

{
    ObjectId = new EntityReference(Email.EntityLogicalName, _emailId),
    ObjectTypeCode = Email.EntityLogicalName,
    Subject = String.Format("Sample Attachment {0}", i),
    Body = System.Convert.ToBase64String(
            new ASCIIEncoding().GetBytes("Example Attachment")),
    FileName = String.Format("ExampleAttachment{0}.txt", i)
};

_emailAttachmentId[i] = _serviceProxy.Create(_sampleAttachment);


Retrieve an attachment
ActivityMimeAttachment _singleAttachment =

    (ActivityMimeAttachment)_serviceProxy.Retrieve(
                                ActivityMimeAttachment.EntityLogicalName,
                                _emailAttachmentId[0],
                                new ColumnSet("activitymimeattachmentid",
                                    "subject",
                                    "filename",
                                    "body"));


Update an attachment
_singleAttachment.FileName = "ExampleAttachmentUpdated.txt";

_serviceProxy.Update(_singleAttachment);


Retrieve all attachments associated with the email activity

QueryExpression _attachmentQuery = new QueryExpression
{
    EntityName = ActivityMimeAttachment.EntityLogicalName,
    ColumnSet = new ColumnSet("activitymimeattachmentid"),
    Criteria = new FilterExpression
    {
        Conditions =
        {
            new ConditionExpression
            {
                AttributeName = "objectid",
                Operator = ConditionOperator.Equal,
                Values = {_emailId}
            },
            new ConditionExpression
            {
                AttributeName = "objecttypecode",
                Operator = ConditionOperator.Equal,
                Values = {Email.EntityLogicalName}
            }
        }
    }
};

EntityCollection results = _serviceProxy.RetrieveMultiple(
    _attachmentQuery);


Delete an Attachment
_serviceProxy.Delete(ActivityMimeAttachment.EntityLogicalName, _emailAttachmentId[0]);

Complete Code
using (OrganizationServiceProxy _serviceProxy = new OrganizationServiceProxy(serverConfig.OrganizationUri,

                                                    serverConfig.HomeRealmUri,
                                                    serverConfig.Credentials,
                                                    serverConfig.DeviceCredentials))
{
    _serviceProxy.ServiceConfiguration.CurrentServiceEndpoint.Behaviors.Add(new ProxyTypesBehavior());

    // Create Email Activity
    Email email = new Email
    {
        Subject = "This is an example email",
        ActivityId = Guid.NewGuid()
    };
    Guid _emailId = _serviceProxy.Create(email);


    Guid[] _emailAttachmentId = new Guid[3];
    // Create three e-mail attachments
    for (int i = 0; i < 3; i++)
    {
        ActivityMimeAttachment _sampleAttachment = new ActivityMimeAttachment
        {
            ObjectId = new EntityReference(Email.EntityLogicalName, _emailId),
            ObjectTypeCode = Email.EntityLogicalName,
            Subject = String.Format("Sample Attachment {0}", i),
            Body = System.Convert.ToBase64String(
                    new ASCIIEncoding().GetBytes("Example Attachment")),
            FileName = String.Format("ExampleAttachment{0}.txt", i)
        };

        _emailAttachmentId[i] = _serviceProxy.Create(_sampleAttachment);
    }

                   

    // Retrieve an attachment including its id, subject, filename and body.
    ActivityMimeAttachment _singleAttachment =
        (ActivityMimeAttachment)_serviceProxy.Retrieve(
                                    ActivityMimeAttachment.EntityLogicalName,
                                    _emailAttachmentId[0],
                                    new ColumnSet("activitymimeattachmentid",
                                        "subject",
                                        "filename",
                                        "body"));


    // Update attachment
    _singleAttachment.FileName = "ExampleAttachmentUpdated.txt";
    _serviceProxy.Update(_singleAttachment);


    // Retrieve all attachments associated with the email activity.
    QueryExpression _attachmentQuery = new QueryExpression
    {
        EntityName = ActivityMimeAttachment.EntityLogicalName,
        ColumnSet = new ColumnSet("activitymimeattachmentid"),
        Criteria = new FilterExpression
        {
            Conditions =
            {
                new ConditionExpression
                {
                    AttributeName = "objectid",
                    Operator = ConditionOperator.Equal,
                    Values = {_emailId}
                },
                new ConditionExpression
                {
                    AttributeName = "objecttypecode",
                    Operator = ConditionOperator.Equal,
                    Values = {Email.EntityLogicalName}
                }
            }
        }
    };

    EntityCollection results = _serviceProxy.RetrieveMultiple(
        _attachmentQuery);

    // Delete an Attachment
    _serviceProxy.Delete(ActivityMimeAttachment.EntityLogicalName, _emailAttachmentId[0]);
}