Currently, there is no constraint between On and wrote: _MULTI_QUOTE_HDR_REGEX = r'(?!On.*On\s.+?wrote:)(On\s(.+?)wrote:)'.

So, even if there is a million characters between On ... blah blah blah ... wrote: it still detects as a multiline quote.

I think the gap between On and wrote: needs to be controlled.

I discovered this bug when the library took 1 minute to parse a long email with multiple On and wrote.

I'm creating a PR for this.