Iptables – How to Block Ports with Telnet and Iptables

I have a Debian server from which I want to block all incoming and outgoing ports (especially all outgoing email ports), except those that I specifically add in iptables. The current setup I have is working and seems restrictive enough, but when I start up a telnet session to test for example port 25 for smtp it just seems to connect without problems, even though I did not specifically open port 25 as OUTPUT port. I noticed that when I flush the iptables with iptables -F, the connection is blocked but then DNS resolution and http traffic also don't work. It might be important to mention that I also run Docker on this server, which means that Docker maintains its own iptables chains which I won't specifically list in this question. None of the Docker rules have port 25 in them, but maybe Docker sets the overall rules too loose?

I haven't found an easy way to actually try to send email to smtp port 25, but I guessed that successfully establising a telnet session would also allow the same connection to be used for sending mail.

Might anyone be able to point me in the right direction?

The telnet output:

$ telnet smtp-relay.gmail.com 25
Trying 2a00:1450:4013:c03::1c...
Connected to smtp-relay.gmail.com.
Escape character is '^]'.
220 smtp-relay.gmail.com ESMTP k6sm844273wms.37 - gsmtp

INPUT and OUTPUT chains of my current iptables:

Chain INPUT (policy DROP)
target     prot opt source               destination
DROP       all  --  anywhere             anywhere             match-set blacklist src
ACCEPT     icmp --  192.168.4.0/24       anywhere             icmp echo-request
ACCEPT     tcp  --  192.168.4.0/24       anywhere             tcp dpt:ssh
ACCEPT     tcp  --  anywhere             anywhere             multiport dports http,https state RELATED,ESTABLISHED
ACCEPT     icmp --  anywhere             anywhere             icmp echo-reply state ESTABLISHED
ACCEPT     tcp  --  anywhere             anywhere             tcp spt:domain state RELATED,ESTABLISHED
ACCEPT     udp  --  anywhere             anywhere             udp spt:domain state RELATED,ESTABLISHED
ACCEPT     tcp  --  anywhere             anywhere             multiport sports http,https state RELATED,ESTABLISHED

Chain OUTPUT (policy DROP)
target     prot opt source               destination
ACCEPT     udp  --  anywhere             anywhere             udp dpt:domain
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:domain
ACCEPT     icmp --  anywhere             anywhere             icmp echo-request
ACCEPT     icmp --  anywhere             192.168.4.0/24       icmp echo-reply state RELATED,ESTABLISHED
ACCEPT     tcp  --  anywhere             192.168.4.0/24       tcp spt:ssh state RELATED,ESTABLISHED
ACCEPT     tcp  --  anywhere             anywhere             multiport dports http,https