Following on from the discussion in Mail problems, and given how easy it is to obtain free of charge SSL certificates these days from the likes of letsencrypt, maybe it’s time to consider the option (possibly with an additional payment) of allowing users to set a custom host name for their client area under a domain they control, e.g. accounts.mycompany.com instead of mycompany.quickfile.co.uk (setting a CNAME in their domain’s DNS).
This should help ameliorate some of the problems with invoice emails being treated as spam, if the from address and the link target both point to the same domain, owned by the business in question rather than by a third party.