I'm kind of hesitating if I should use a subdomain or just a plain word as hostname. E.g. my domain name is domain.com and my server's hostname ip1.domain.com.
This might be a strange question but the reason why I'm hesitating is because I wonder if the hostname has any influence on mail delivery reliability.
You can call it whatever you like. The only real importance is consistency. Especially when it comes to ensuring mail deliverability.
Take care to ensure that the hostname reported by your MTA's HELO, the forward A DNS record and the reverse PTR DNS record are all the same.
Your hostname, especially for email, can be exposed to the real-world in the form of headers, but in general it has no real impact on mail delivery. However, just make sure you don't set your hostname to domain.com if that is a domain you are accepting mail for. Postfix uses the hostname to set particular variables and the default option is to deliver mail locally for the hostname.
If your hostname is localhost[.xxx.yyy], you might have an hard time having mail delivered to it ...
No, it's just cosmetic. If you're not careful, many programs (postfix I think, apache, etc.) will use whatever "hostname" reports, which can cause problems. Otherwise, there's nothing magical about the hostname.
mail delivery depends on lookup of a dns record type defined as MX. There is no link between the shortness of the name and the ability of it to be looked up.
