Single use emails are ... single use

[wttw]

At least one ESP does something odd when sending a test email - it doesn't send mail as it would be sent to recipients, rather it sends two emails, one with the text/plain part and one with the text/html part.

I can see why they might want to do that (even though it's probably the only time anyone will ever see the text/plain part) but the first text/plain version gets delivered to us, which disables the email address and the text/html version then bounces.