Hello @zon and welcome to the community!
You described two different situations in the thread above.
The situation you described above assumes something goes wrong on the server or there is a server-side issue, while the situation you described below assumes something goes wrong on your phone.
If there is a server issue which makes your current password stop working for whatever reason but everything is still working on your phone then it might be better to use AEAP to continue messaging contacts. You can easily continue chatting with your contacts using a new email address while keeping the same encryption and identity key.
However if something goes wrong on your phone, then you need a different approach. The email server doesn’t store any account or contact information so there’s nothing to “retrieve” from there except possibly new/unread messages.
As Simon says, all your contacts and encryption keys are stored on your phone, so it’s advisable to make a backup, although Delta Chat currently doesn’t encrypt it so you better make sure to encrypt it yourself if you plan to upload your backup or transfer it over an insecure channel. I use OpenKeychain to encrypt backups on Android, but unfortunately this app is not maintained. Backups can also be unnecessarily large because they also include all the media from your chats, but I hope that improvements to the backup feature are being considered to address these issues.
You should keep backups up-to-date especially after adding new contacts.
I have also suggested a feature to request contacts to automatically resend encryption keys, which I think could help to recover contacts in specific circumstances when your backup is not up-to-date, but this won’t apply to every situation.