input

Hey man. My flag got kidnapped, can you help me get it back?
Warning: do not use yopmail.com w/ this service. Attachment

output

We tried to send an email with the 5ymail.com service. We received the following credentials:
Username = 115261-eQAWO / Password = eQAWOBI6

By looking through the mail headers from our anonymous mail sent to ourself, we remarked this line:
Your_FAKE_Name.eQAWOBI6Y9255397-115261-EN@5ymail.com

So we retrieved this line from the attachment of the challenge:
Bad_Guy.nUWAQuXA8i253005-114196-EN@5ymail.com

And we deduced the Bad Guy credentials:
Username = 114196-nUWAQ / Password = nUWAQuXA

We connected to the service with these credentials and we obtained: