@martinbook85 : Every mailbox has an address. Many providers offer the option to create additional alias addresses to this main address. If someone sends a mail to this alias address, this mail is also stored and available to the owner of the mailbox in the same way as mails to the main address. Sorting/filtering must be done by mail clieent, e.g. Thunderbird.
Technically, an alias address is, that on the mail server 1. the alias address is set up as a valid address, but without an own inbox mailspace, and 2. that mails to the alias address is stored into the inbox of the main address, and 3. that mails with sender = alias address are not filtered as spam when sending mails via the SMTP server of the main address.
If you want to send a mail from an alias address, you must have several ‘identities’ set up in your mail client. An identity is an alternative set of personal data like real name, senders address, reply-to address, footer text and so on. SFOS Mail client isn’t able to do this.
GMX allows it. There you can set up alias addresses via the webmail interface, in the personal settings.