If the attacker has ever seen the headers of a message you sent through fastmails SMTP service they have your public IP (Received from header) and can Dos you directly anyway.

They do something similar with their webmail service, but the data is encrypted so it can't be read by a third party.

https://www.fastmail.com/about/reportabuse.html (last paragraph)

EDIT: Fastmail is fairly priced (for me) and i like the features they offer but i wish they wouldn't do this (or rather, i wish they would do the same for the SMTP service as they do for the webmail service)