Hi, I'm using Kmail 5.2.0 and I wish I could use blockquote tag in html replys instead of indenting so I would be compatible with other clients. The problem with indenting is also that it breaks formating of original e-mail. Partial support for blockquote is in KMail already. Incoming e-mails from other clients using blockquote are displayed correctly but in composer when replaying all blockquotes are transformed into indents which breaks original formating and compatibility with other clients.
I am in support of this feature. If this feature is implemented, I would prefer to have <blockquotes> not only for replying or relaying other mails but also to be able to insert custom <blockquotes> with the tool bar. I often discuss 3rd party literature via mail and having a feature like this would help to distinguish between my own text and the citation. There is a loosely related feature request that asks for real left indention of paragraphs without unintentionally creating a bullet list. See bug #341961.