Some emails are putting plaintext that should be in the body as an attachment

Hi @Nathan McMinn,

The issue seems to be related to the MIME formatting of the emails being sent through Azure Communication Services (ACS) via SMTP. When emails are sent in plain text but without proper Content-Type headers (or if there's ambiguity in how the message is structured), email clients may misinterpret the body as an attachment, typically as a .txt file. This behavior can change if there's an update on the recipient’s mail system or ACS’s handling of MIME content, even if your application code hasn’t changed.

When constructing your email, specify the content type and encoding to guide email clients in rendering the message appropriately.

• Content-Type: Set to text/plain; charset=utf-8.
• Encoding: Use UTF-8 encoding for the email body. https://learn.microsoft.com/en-us/exchange/mail-flow/content-conversion/content-conversion?view=exchserver-2019#content-type-header-field

If you're using a programming language like C#, utilize the System.Net.Mail namespace to construct your email messages. Here's an example:

using System.Net.Mail;
using System.Text;
var mail = new MailMessage();
mail.From = new MailAddress("[email protected]");
mail.To.Add("[email protected]");
mail.Subject = "Test Email";
mail.Body = "This is a plain text message.";
mail.IsBodyHtml = false;
mail.BodyEncoding = Encoding.UTF8;
using (var smtpClient = new SmtpClient("smtp.azurecomm.net", 587))
{
    smtpClient.Send(mail);
}

This code ensures that the email body is treated as plain text with UTF-8 encoding, reducing the likelihood of it being misinterpreted as an attachment.

Kindly refer to the below documentations for better understanding:
https://learn.microsoft.com/en-us/exchange/mail-flow/content-conversion/message-encoding?view=exchserver-2019
https://learn.microsoft.com/en-us/dotnet/api/system.net.mail.mailmessage?view=net-8.0
https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/send-email-smtp/send-email-smtp?pivots=smtp-method-smtpclient

I hope you find this information useful. Let me know if you have any further questions.