I am developing a Windows Background Service with .NET and C#. It periodically queries a REST API and sends emails if certain conditions are met. Sometimes the data returned from the API will have image files and sometimes not. When image files are returned, the service embeds them into the email body before sending. I'm embedding the image file in four steps:

1. Download the image as a byte[]
2. Convert byte[] to MemoryStream
3. pass the stream to a new LinkedResource
4. Add the linked resource to the AlternateView for the MailMessage

This works if the message has a single "To" recipient and no CC or BCC recipients. The image displays correctly in the body of the email. The weird problem I'm having is that if I add multiple recipients in To, CC or BCC, the image fails to display in the email body. This problem seems to be limited to Outlook. I did a test with two recipients, one internal to the organization and one Gmail account. The image displayed correctly in Gmail but was broken in Outlook.

Here is the snippet of code where the service builds the LinkedResource and sends the email.

htmlBody = htmlBody.Replace("{incident_time}", incident.IncidentTime)
    .Replace("{incident_type}", incident.IncidentType)
    .Replace("{incident_location}", incident.IncidentLocation)
    .Replace("{reporting_agency}", incident.ReportingAgency)
    .Replace("{severity}", incident.Severity)
    .Replace("{details}", incident.Details);

// embed attached images
var attachmentsResult = await arcGisRestClient.QueryAttachmentsAsync(
    incidentSublayerUrl,
    tokenResponse.Token,
    objectId: incident.ObjectId);

IList<LinkedResource> linkedResources = new List<LinkedResource>();

if (!string.IsNullOrEmpty(attachmentsResult))
{
    var attachmentsObj = JObject.Parse(attachmentsResult);
    var attachments = FeatureAttachmentConverter.FromJObject(attachmentsObj);
    // if attachments > 0
    if (attachments.Count > 0)
    {
        string imgHtml = "";

        foreach (var attachment in attachments)
        {
            string attachmentUrl = $"{incidentSublayerUrl}/{attachment.ParentObjectId}/attachments/{attachment.Id}?token={tokenResponse.Token}";
            WebClient webClient = new WebClient();
            byte[] bytes = webClient.DownloadData(attachmentUrl);
            MemoryStream stream = new MemoryStream(bytes);
            LinkedResource linkedResource = new LinkedResource(stream, MediaTypeNames.Image.Jpeg);
            imgHtml += @"<img src='cid:" + linkedResource.ContentId + @"' width='450'/>";
            linkedResources.Add(linkedResource);
        }

        htmlBody = htmlBody.Replace("{attachments}", imgHtml);
    }
    else
    {
        htmlBody = htmlBody.Replace("{attachments}", "");
    }
}

AlternateView htmlAlternate = AlternateView.CreateAlternateViewFromString(htmlBody, new ContentType(MediaTypeNames.Text.Html));
foreach (var lr in linkedResources)
{
    htmlAlternate.LinkedResources.Add(lr);
}

// create email
MailMessage message = new MailMessage
{
    Subject = "TEST Incident Notification",
};

message.AlternateViews.Add(htmlAlternate);

string[] emailToAddresses = _config.GetSection("EmailNotificationTo").Get<string[]>();
message.To.Add(string.Join(',', emailToAddresses));

string[] emailCcAddresses = _config.GetSection("EmailNotificationCc").Get<string[]>();
message.CC.Add(string.Join(',', emailCcAddresses));

message.From = new MailAddress(_config["EmailNotificationFrom"]);

// send email
SmtpClient smtpClient = new SmtpClient("mail.domain.org", 25);
using (smtpClient)
{
    smtpClient.Send(message);
    logger.Info($"Email notification sent; objectid: {incident.ObjectId}");
}

Could this be an issue with our internal STMP server or Outlook settings rather than a problem with the above code?