I am getting this issue on reverse DNS in one server:
Reverse DNS is not correctly configured for IPv4. Some emails may fail to get delivered or be flagged as spam.
Current reverse DNS: dynamic-adsl-xx-xxx-xxx-xxx.client.isp.com
Expected value: your domain
You should first try to configure reverse DNS with your domain in your internet router interface
how to configure it inside the openwrt?
What server ?
How are you sending emails ?
Do you have an SMTP set up on your router ?
You don't configure this "inside the OpenWrt". This is a global setting which requires you to control the global DNS PTR Record. Those records are controlled by the ISP.
This is one method employed that prevents standard ISP customers from setting up mail servers. The other common method is that ISPs block use of port 25/tcp.
Since you're inquiring here, I surmise that you don't have your ISP's permission to setup an email server.
This implies the IP address is dynamic and not static. While it's possible to use a dynamic IP, it's been global practice for years in a lot of ISPs to block SMTP on thier dynamic non-business-account IP ranges.
port 25 is open and my isp is not blocking it. the only problem i have is with reverse dns
OK, here are the steps:
https://bgp.he.net/ip/<ip_address>
It will tell you who the IP is registered to.
Contact that entity and ask that they change the PTR record corresponding to your IP, from dynamic-adsl-xx-xxx-xxx-xxx.client.isp.com - to the FQDN of your mail server.
Success and joy.
Or ask them to provide you with a static IP, as long as it's dynamic, you might have to email them .... often.
that's an option, but even getting static IP, it does not mean they will give me reverse DNS automatically
I forgot to note to the OP, ISPs usually won't change a PTR record unless they've issued a static IP.
True, but then you can register your own FQDN pointing to the IP they've given you.
It's usually required you have been assigned a static IP to set a PTR record.
Unless I misunderstood you - no the OP cannot. The xxx.xxx.xxx.xxx.in-addr.arpa zone is controlled by the ISP.
is it done inside my router?
No, see post:
on my other server with static IP at my office, I had to ask the ISP for the reverse DNS and they provide it.
No, and probably not free.
Correct, the same concept applies on any ISP.
so maybe a vps just to forward the traffic with wireguard would be better(and cheaper)
Shouldn't be an issue though...
You can have several FQDNs pointing to the same IP ?
You still need an endpoint with a Public IP address that the ISP allows you to change the PTR record.
Not sure why you think that, but OK.
OP needs a Reverse PTR record (the ISP controls that zone), you can only have 1 such record per IP. I think you are confusing the Forward Record (i.e. A Record).
Then lets hope the ISP haven't configured one 
But at least it's easy to check/ask.
Works for my ISP, but they also give out static ipv4s by default, so slight apple and banana...
They have, that the OP's issue: