H

Hard Bounce:
A hard bounce is the failed delivery of an e-mail due to a permanent reason, such as a non-existent e-mail address.

Harvesting Emails:
Extracting emails from pages on the internet using a robot software. Either through a search starting from a single page or a search based on a specific term put into a search engine. Used in email spamming.

HTML e-mail:
An html formatted e-mail instead of plain text. HTML allows a graphical appearance with fonts, graphics and background colors. HTML makes an e-mail more interesting and can improve the response rates of a campaign.
Header

Routing and program data at the start of an email message, including the sender's name and email address, originating email server IP address, recipient IP address and any transfers in the process.

High Level Domain:
The portion of the Domain that is to the right of the last period, e.g. "com, net, gov". This is useful for checking email validity.