After a while i was not able to send emails again.
I used the add or remove components tool in plesk to remove postfix and installed qmail.
Then i did it the other way around. I guess like 
@fhe did it.
After this i was able to send emails again.
Heres's what i found out:
1. No Change here:
cat /etc/postfix/sasl/smtpd.conf output:
pwcheck_method: auxprop saslauthd
auxprop_plugin: plesk
saslauthd_path: /private/plesk_saslauthd
mech_list: DIGEST-MD5 CRAM-MD5 PLAIN LOGIN
sql_engine: intentionally disabled
log_level: 4
2. No change here:
stat /var/spool/postfix/plesk/passwd.db output:
  File: /var/spool/postfix/plesk/passwd.db
  Size: 36864           Blocks: 72         IO Block: 4096   regular file
Device: 45b0b601h/1169208833d   Inode: 2228420     Links: 1
Access: (0460/-r--rw----)  Uid: (  105/ postfix)   Gid: (    0/    root)
Access: 2021-03-20 00:54:23.178134500 +0100
Modify: 2021-03-20 00:54:22.644133694 +0100
Change: 2021-03-20 00:54:22.644133694 +0100
 Birth: -
3. This Line changed  in the file /etc/postfix/master.cf from:
smtp      inet  n       -       -       -       -       smtpd
to
smtp      inet  n       -       y       -       -       smtpd
I think this Solution is Similar to 
@Andri 
But it's not the same.
Hope it keeps working now. And hope it finally helps you all.
All active Lines in master.cf, just in case i missed a change you can compare:
smtp      inet  n       -       y       -       -       smtpd
cleanup   unix  n       -       y       -       0       cleanup
tlsmgr    unix  -       -       y       1000?   1       tlsmgr
rewrite   unix  -       -       y       -       -       trivial-rewrite
bounce    unix  -       -       y       -       0       bounce
defer     unix  -       -       y       -       0       bounce
trace     unix  -       -       y       -       0       bounce
verify    unix  -       -       y       -       1       verify
flush     unix  n       -       y       1000?   0       flush
proxymap  unix  -       -       n       -       -       proxymap
proxywrite unix -       -       n       -       1       proxymap
smtp      unix  -       -       y       -       -       smtp
relay     unix  -       -       y       -       -       smtp
showq     unix  n       -       y       -       -       showq
error     unix  -       -       y       -       -       error
retry     unix  -       -       y       -       -       error
discard   unix  -       -       y       -       -       discard
local     unix  -       n       n       -       -       local
virtual   unix  -       n       n       -       -       virtual
lmtp      unix  -       -       y       -       -       lmtp
anvil     unix  -       -       y       -       1       anvil
scache    unix  -       -       y       -       1       scache
postlog   unix-dgram n  -       y       -       1       postlogd
plesk_virtual unix - n n - - pipe flags=DORhu user=popuser

opuser argv=/usr/lib/plesk-9.0/postfix-local -f ${sender} -d ${recipient} -p /var/qmail/mailnames
127.0.0.1:12346 inet n n n - - spawn user=popuser

opuser argv=/usr/lib/plesk-9.0/postfix-srs
mailman unix - n n - - pipe flags=R user=list:list argv=/usr/lib/plesk-9.0/postfix-mailman ${nexthop} ${user} ${recipient}
pickup fifo n - y 60 1 pickup
qmgr fifo n - n 1 1 qmgr
smtps inet n - y - - smtpd -o smtpd_tls_wrappermode=yes
plesk_saslauthd unix y y y - 1 plesk_saslauthd status=5 listen=6 dbpath=/plesk/passwd.db