Reverse DNS is not a valid Hostname

My YunoHost server

Hardware: VPS bought online
YunoHost version: 11.0.11
I have access to my server : Through SSH
Are you in a special context or did you perform some particular tweaking on your YunoHost instance ? : no

Description of my issue

Getting “Reverse DNS is not a valid Hostname” for SMTP testing via MXToolbox.

From my searches online, all signs point to me needing to update my hostname to not be my main domain especially for SMTP and spam filtering purposes (a la, but Yunohost does not seem to like this methodology? Why is that? I had read that this isn’t advisable, but maybe that’s outdated.

Is there a way to change my hostname?

I am dealing with my emails bouncing from Microsoft’s servers. I’ve already put in a support request via their form, and made sure my security information is set up (SPF, DMARC, and DKIM) correctly, and I have verified my domain with Google (emails were going to spam), so I’m trying to tie up loose ends at this point. Any advice would be appreciated.

Well I’m confused about your message … To me the issue lies in the reverse DNS itself, not in your hostname … Did you actually configure the reverse DNS ?

I did–the only time I have mxtoolbox behave correctly re: this is if I create a fake subdomain that points to my IP address via my domain name provider, and then make the reverse DNS reflect that, and then I get a different error re: my SMTP banner not matching my hostname.

I know I can change the file details for that in postfix, but since there’s no defined hostname that’s listed as, it’s kind of… a fluke?

Well then idk, is there anything special with your hostname / main domain ? Is it a subdomain ? Or maybe a funky TLD ?

My main domain has a .co suffix, but there shouldn’t be anything funky with it past that. There’s no subdomain involved re: my main hostname/domain.

I’m familiar enough with all of this to follow instructions, but not enough to make major modifications to the way yunohost is set up.

Then I don’t know why MXToolbox would complain about that … Does any other tool complain about it ?

From this thread:

« Your PTR record is which is a Domain name. The RFC guidelines say that is must be a hostname such as Most systems won’t care that it is a domain, but since it’s against the RFC guidelines, we show a warning. »

Honestly I don’t quite know who really enforces that hostname should be mail.domain.tld, to me this sounds like bullshit …

Alright. Does that mean I should ignore the warning, then?

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