Enabling spammers.
Our company has started receiving a huge amount of automated spam from firemail.de freemail accounts.
We have had to block the entire firemail.de domain as a sender because Firemail does not do a good job of screening new accounts to prevent abuse (this is why the spammers are using firemail.de addresses instead of gmail or outlook).
As a firemail user this means that you will have trouble getting your emails delivered because the email infrastructure provider has a bad reputation.
As a company, you have no choice but to block the firemail service from delivering mail to your domains.
If firemail wants any chance of surviving, they need to employ some basic KYC to verify accounts, prevent duplicate accounts and also detect suspicious/automated/spammy behaviour on outgoing mail from accounts. If they don't do this, their reputation will continue to get worse and worse until the service is completely unusable due to inability to deliver mail. Until they get their act together, this freemail service is a great choice for spammers to abuse...as many companies may have not yet blocked the service.