Postfix error with remote mailserver – Error 550

i am new to mailservers and i think i totaly destroyed the config and i have no clue how to fix this mess.
For my understanding things are "OK" but i think i'm missing something important.

I replaced IPs with SERVER1 and SERVER2 and the domain with MYDOMAIN.

Setup:

2 Hetzner Server: One for Website (SERVER1 – VPS), another one for mails and other stuff (SERVER2 – Webhosting)
The website needs to send smtp mails with PHP and this part is working. But if the user mail is user@MYDOMAIN.com, it wont be delivered and i get the errors:

status=bounced (host mail.MYDOMAIN.com[SERVER2] said: 550 Sender verify failed (in reply to MAIL FROM command))

status=bounced (host mail.MYDOMAIN.com[SERVER2] said: 550 Unroutable address (in reply to RCPT TO command))

I tried so many different things and configurations but neither of them works.
Here is where it is at right now.

DNS:

$TTL 7200
@   IN SOA ns1.your-server.de. postmaster.your-server.de. (
    2018033101   ; serial
    86400        ; refresh
    10800        ; retry
    3600000      ; expire
    86400 )      ; minimum

@                        IN NS      ns3.second-ns.de.
@                        IN NS      ns.second-ns.com.
@                        IN NS      ns1.your-server.de.

@                        IN A       SERVER1
entwickler               IN A       SERVER2
mail                     IN A       SERVER2
www                      IN A       SERVER1
autoconfig               IN CNAME   mail.your-server.de.
ftp                      IN CNAME   www
imap                     IN CNAME   mail
pop                      IN CNAME   mail
relay                    IN CNAME   mail
smtp                     IN CNAME   mail
@                        IN MX 10   mail
_autodiscover._tcp       IN SRV     0 100 443 mail.your-server.de.
@                        IN TXT     "v=spf1 +a +mx ?all"

Hostname SERVER1:

hostname -> mail
hostname -f -> mail.MYDOMAIN.com

Postfix config:

smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no
append_dot_mydomain = no

readme_directory = no

# TLS parameters
smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache

smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = mail.MYDOMAIN.com
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = MYDOMAIN.com, localhost
relayhost =
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
inet_protocols = all

I am very thankful for any kind of advice and help you guys can give me.
I'm trying to learn but mailservers are a real diva…

Let me know if you need some more informations.

UPDATE:

After I changed the origin and destination the error changed to something i already had before:

status=bounced (unknown user: "USER")

Here is the full log:

Apr  5 09:09:19 mail postfix/pickup[29043]: B30A82645F9: uid=33 from=<www-data>
Apr  5 09:09:19 mail postfix/cleanup[29052]: B30A82645F9: message-id=<20180405070919.B30A82645F9@mail.MYDOMAIN.com>
Apr  5 09:09:19 mail postfix/qmgr[29044]: B30A82645F9: from=<www-data@MYDOMAIN.com>, size=426, nrcpt=1 (queue active)
Apr  5 09:09:19 mail postfix/local[29054]: B30A82645F9: to=<USER@MYDOMAIN.com>, relay=local, delay=0.04, delays=0.02/0/0/0.01, dsn=5.1.1, status=bounced (unknown user: "USER")
Apr  5 09:09:19 mail postfix/cleanup[29052]: BAA252645FC: message-id=<20180405070919.BAA252645FC@mail.MYDOMAIN.com>
Apr  5 09:09:19 mail postfix/bounce[29055]: B30A82645F9: sender non-delivery notification: BAA252645FC
Apr  5 09:09:19 mail postfix/qmgr[29044]: BAA252645FC: from=<>, size=2296, nrcpt=1 (queue active)
Apr  5 09:09:19 mail postfix/qmgr[29044]: B30A82645F9: removed
Apr  5 09:09:19 mail postfix/local[29054]: BAA252645FC: to=<www-data@MYDOMAIN.com>, relay=local, delay=0.02, delays=0.01/0/0/0.01, dsn=2.0.0, status=sent (delivered to mailbox)
Apr  5 09:09:19 mail postfix/qmgr[29044]: BAA252645FC: removed

Mails to gmail and so on are still working without errors.
I guess he's still trying to attach the mail to the localhost instead passing it over to the mail server?

UPDATE 2:

I now changed mydestination to localhost since this seems to be right?
Now the relay is the mailserver but 2 new errors show up:

Apr  5 10:18:45 mail postfix/pickup[29400]: 4BB7F264615: uid=33 from=<www-data>
Apr  5 10:18:45 mail postfix/cleanup[29425]: 4BB7F264615: message-id=<20180405081845.4BB7F264615@mail.MYDOMAIN.com>
Apr  5 10:18:45 mail postfix/qmgr[29401]: 4BB7F264615: from=<www-data@MYDOMAIN.com>, size=426, nrcpt=1 (queue active)
Apr  5 10:18:48 mail postfix/smtp[29426]: 4BB7F264615: to=<USER@MYDOMAIN.com>, relay=mail.MYDOMAIN.com[SERVER2]:25, delay=3.1, delays=0.02/0/3.1/0, dsn=5.0.0, status=bounced (host mail.MYDOMAIN.com[SERVER2] said: 550-Verification failed for <www-data@MYDOMAIN.com> 550-Unrouteable address 550 Sender verify failed (in reply to MAIL FROM command))
Apr  5 10:18:48 mail postfix/cleanup[29425]: 73183264624: message-id=<20180405081848.73183264624@mail.MYDOMAIN.com>
Apr  5 10:18:48 mail postfix/bounce[29438]: 4BB7F264615: sender non-delivery notification: 73183264624
Apr  5 10:18:48 mail postfix/qmgr[29401]: 73183264624: from=<>, size=2611, nrcpt=1 (queue active)
Apr  5 10:18:48 mail postfix/qmgr[29401]: 4BB7F264615: removed
Apr  5 10:18:51 mail postfix/smtp[29426]: 73183264624: to=<www-data@MYDOMAIN.com>, relay=mail.MYDOMAIN.com[SERVER2]:25, delay=3, delays=0.01/0/3/0.02, dsn=5.0.0, status=bounced (host mail.MYDOMAIN.com[SERVER2] said: 550 Unroutable address (in reply to RCPT TO command))
Apr  5 10:18:51 mail postfix/qmgr[29401]: 73183264624: removed