Strip tags that don't add value when a forum post is converted into a plain text email message:

• [/url] tag breaks links when the email client interprets the closing tag as part of the link (example 1)
• [/img] doesn't render the image inline, it just breaks the link (example 2)
• [b], [u], [i] and [s] are not converted into bold, underline, italics and strikethrough so they merely add noise and make the text harder to read (example 3)

The [quote] and [code] tags arguably add some value as they tell the reader that the enclosed text should be consider a quote or code. You can leave those.

In general the tags seem to add little value unless the post is sent as HTML and the tags are converted to their HTML equivalents.