We do have Leads that came through Marketo forms and contain spaces, because someone forgot to change the field type from the default 'text' to 'email' on the form.
If you want to flag those existing leads as probable typos (can't be absolutely sure, since in extremely rare cases people are using legit emails with spaces) a FlowBoost one-liner will pick them up:
var emailHasSpaces = /\s/.test({{Lead.Email Address}});