Webmail Filter Criteria

[hurricane_dodger]

When creating a mail message filter one of the options is to select "String" as a value to be searched/used and a variety of criteria (e.g. "contains", "matches expression", "matches regular expression", etc.) can be specified. What type of value is valid in the "String" field? Is it something like a Mail Header (e.g. "Reply-To:") or am I missing the point? Please advise because I have attempted to match a mail header and I've not been successful.