Why specify host for SMTP?

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http://serverfault.com – I'm sending emails using gmail's SMTP API. You can specify the host. I don't yet have a domain name so have set this as the IP of my production server. I presume setting the host makes it less likely your messages will be marked as spam and if they are you can resolve any blocking if your messages are legitimate? Any further sources as to understanding what the host attribute is for would be appreciated. (HowTos)