Hi Gautier,
As Daniela mentioned, you need to include the following namespace:
using Skyline.DataMiner.Net.Messages;

Please find below a full example:
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using System;
using System.Collections.Generic;
using System.Globalization;
using System.IO;
using System.Text;
using Skyline.DataMiner.Automation;
using Skyline.DataMiner.Net.Messages;

class Script
{
public void Run(Engine engine)
{
string CSVPath = @”C:Skyline DataMinerDocumentsDMA_COMMON_DOCUMENTSTEST_REPORTStestReport.csv”;
EmailAttachment myAttachement = new EmailAttachment(“CPUReport.csv”, File.ReadAllBytes(CSVPath));
SendEmailMessage myMessage = new SendEmailMessage()
{
Attachments = new EmailAttachment[1] { myAttachement },
From = “Reports@DataMiner.be”,
To = “your.email@skyline.be”,
Subject = “CPU Report”,
Body = “Find attached the test report”
};
engine.SendSLNetMessage(myMessage);
}
}

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Disclaimer about Skyline.DataMiner.Net.Messages:
Note that this is an internal call and we do not recommend using this, as it is not officially supported and we cannot guarantee that it will still work in the future. As a rule, you should avoid using SLNet calls, as these are subject to change without notice. We recommend to instead always use the correct UI or automation options provided in DataMiner Automation or through our web API.