And if someone wants to DDoS you, you're a lot more vulnerable than a major provider like Fastmail.
Personally, I use a hybrid solution: I use Gandi's SMTP servers for outbound and inbound mail, but I run my own IMAP server for unlimited storage under my control.
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)
Personal users aren't of interest to DOS ransomers since a call to the ISP will drop their traffic at the border. Without SLAs costing me money, as would be the case with a big provider, I coulf outlast the DOS. Just inconvenient and annoying.