In an ideal world, our data would always be backed up to the last minute, but in practice, I think most of us only back up our data occasionally.
If someone recently added some contacts, and then their phone breaks or they lose their phone and find they need to restore from a backup created at a previous point in time (before they added the recent contacts), they will not be able to send messages to these contacts if they use a Chatmail account, because they won’t have the keys for these contacts and won’t be able to encrypt the messages.
However, if this person knows the addresses of their contacts or has them written down, and Delta Chat has a feature to request resending keys, they could send a special “key resend request” message to these contacts, receive the keys again, and continue messaging normally.
This would still respect the security and anti-spam motivations for restricting Chatmail accounts to encrypted messaging only. Because this feature would only allow Delta Chat to resend keys to established contacts, this feature cannot be abused by spammers. Also the key which is resent could be encrypted using the key of the one who sent the request, guaranteeing the security and integrity of the E2EE process. As the process is secure unless I overlook something, I think the Delta Chat client could automatically respond to a request to resend the key and just display an information message in the chat when the key is resent.