They should be correct; I checked the documentations on how to set them up on the Forums aswell..

I have the fail2ban plugin disabled

Ports should be open aswell, telnet on port 25 works aswell

root@mail:~# iptables -L -n
Chain INPUT (policy DROP)
target     prot opt source               destination
ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0            state RELATED,ESTA                                           BLISHED
ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0
ACCEPT     icmp --  0.0.0.0/0            0.0.0.0/0            icmptype 8
ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:22
ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:80
ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:443
ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:25
ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:587
ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:110
ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:995
ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:143
ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:993

Chain FORWARD (policy DROP)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

My hostname is: mail.dinhware.org and my mail uses @dinhware.org
What am I missing I cant seem to figure it out

Yes I get this

Oct  8 14:37:16 mail postfix/postscreen[23449]: CONNECT from [MY_IP_ADRESS]:58781 to [194.135.95.21]:25
Oct  8 14:37:16 mail postfix/dnsblog[23451]: addr MY_IP_ADRESS listed by domain zen.spamhaus.org as 127.0.0.10
Oct  8 14:37:22 mail postfix/postscreen[23449]: DNSBL rank 3 for [MY_IP_ADRESS]:58781

When I disconnect

Oct  8 14:39:00 mail postfix/postscreen[23449]: HANGUP after 99 from [MY_IP_ADRESS]:58781 in tests after SMTP handshake
Oct  8 14:39:00 mail postfix/postscreen[23449]: DISCONNECT MY_IP_ADRESS]:58781

Not sure, I went with a fresh setup and just took all default settings iRedMail gave me

Just checked my inbox and found a bunch of these messages:

Error when sending messages to the following receivers or groups:

business@dinhware.org (business@dinhware.org)
Your message could not be delivered. Despite repeated delivery attempts for your message, an error occurred while querying the domain name of the recipient in the DNS (Domain Name System).

For more information and tips on how to resolve this issue, visit this article: http://go.microsoft.com/fwlink/?LinkId=389361.

Is there anything wrong with my DNS Settings I posted above?

I have just tried sending an E-Mail to business@mail.dinhware.org just because I was curious
This appeared in my maillog

Oct  8 16:36:25 mail postfix/postscreen[25769]: CONNECT from [40.92.68.78]:58772 to [194.135.95.21]:25
Oct  8 16:36:31 mail postfix/postscreen[25769]: PASS NEW [40.92.68.78]:58772
Oct  8 16:36:31 mail postfix/smtpd[25780]: connect from mail-oln040092068078.outbound.protection.outlook.com[40.92.68.78]
Oct  8 16:36:31 mail postfix/smtpd[25780]: Anonymous TLS connection established from mail-oln040092068078.outbound.protection.outlook.com[40.92.68.78]: TLSv1.2 with cipher ECDHE-RSA-AES256-SHA384 (256/256 bits)
Oct  8 16:36:32 mail postfix/smtpd[25780]: NOQUEUE: reject: RCPT from mail-oln040092068078.outbound.protection.outlook.com[40.92.68.78]: 550 5.1.1 <business@mail.dinhware.org>: Recipient address rejected: User unknown in local recipient table; from=<private@email.adress> to=<business@mail.dinhware.org> proto=ESMTP helo=<EUR02-HE1-obe.outbound.protection.outlook.com>
Oct  8 16:36:32 mail postfix/smtpd[25780]: disconnect from mail-oln040092068078.outbound.protection.outlook.com[40.92.68.78]

Furthermore I got an instant reply back

mail.dinhware.org has rejected your message to the following e-mail addresses.
business@mail.dinhware.org (business@mail.dinhware.org)
The address to which you sent your message was not found in the target domain. It may be misspelled, or it does not exist. Try to resolve the problem by doing one or more of the following:
Send the message again, but before you do this, delete the address and re-enter it. If your e-mail program automatically proposes an address to use, do not select it.
Delete the AutoComplete cache for recipients in your e-mail program by following the steps in this article: Status Code 5.1.1 . Send the message again, but before you do so, delete the address and re-enter it.
To make sure you are using the correct address, contact the recipient in a different way (eg, by telephone). Also, ask if an e-mail forwarding rule is set up that might forward your message to an incorrect address.

Additionaly: I have also setup SSL certificates with Lets-Encrypt for dinhware.org and mail.dinhware.org, not sure if this has to do with the issue

This is the key.

Checked your DNS, the MX record is wrong:

Domain name "mail.dinhware.org.dinhware.org" is not pointed to an IP address, so i think it should be "mail.dinhware.org", but not "mail.dinhware.org.dinhware.org".