I am now using the free tier Oracle cloud account (ARM64) with HestiaCP. They are doing NAT. It looks like the outgoing email not getting to the right IP address.
2022-10-06 00:01:03 Start queue run: pid=399207
2022-10-06 00:03:14 1ofMX7-000bss-W1 H=mail.example.com [112.211.111.118]: SMTP timeout after initial connection: Connection timed out
2022-10-06 00:03:14 1ofMX7-000bss-W1 == [email protected] R=dnslookup T=remote_smtp defer (110): Connection timed out H=mail.example.com [112.211.111.118]: SMTP timeout after initial connection
2022-10-06 00:03:14 1ogEM6-001frL-GL <= <> R=1ofMX7-000bss-W1 U=Debian-exim P=local S=2630
2022-10-06 00:03:14 1ogEM6-001frL-GL remote host address is the local host: host.example.com
2022-10-06 00:03:14 1ogEM6-001frL-GL == [email protected] R=dnslookup defer (-1): remote host address is the local host
2022-10-06 00:03:14 1ogEM6-001frL-GL Frozen
2022-10-06 00:03:14 1ofY3B-000QYQ-G4 == [email protected] R=dnslookup T=remote_smtp defer (-54): retry time not reached for any host for 'example.com'
2022-10-06 00:03:14 1ofMUD-000blC-UB == [email protected] R=dnslookup T=remote_smtp defer (-54): retry time not reached for any host for 'example.com'
2022-10-06 00:03:14 1ofNXN-0000SC-9F Message is frozen
2022-10-06 00:03:14 1ofXSv-000Kz5-Nb == [email protected] R=dnslookup T=remote_smtp defer (-54): retry time not reached for any host for 'example.com'
Also, I have found that there is no webmail icon available in the web interface. It is different from a paid VPS at another VPS hosting provider (directly using public IP).