Subdomains follow the usual domain requirements:
Use only these characters:
- (not as a starting or ending character)
Label must 63 characters ("octets") or less and names must be 255 characters ("octets") or less.
For more information, see RFC 1035.
The subdomains are not registered in the DNS-records.
Especially the last point bothers me. Is it possible to achieve this without registering each subdomain in the DNS records?
The sub domains must exist in DNS because otherwise people can't resolve them.
But you don't need to explicitly create a DNS record for each subdomain, you can create a DNS ...
No issues. Remember, it's always DNS! Meaning, you control what the outside world sees as your mail server through the use of MX records in DNS. When another server is sending an email along that's addressed to email@example.com, the outside DNS server will lookup your MX record in your root domain example.com and find whatever you put in there as the MX ...