I’ve got this as a feature but I’d also say it strays slightly into being a bug too, in so much as it is a part of Quickfile that causes issues.
I have always used my own SMTP service for Quickfile but still get plagued with issues on deliverability because of spam filtering on destination servers. I also get the odd Quickfile invoice sent to me which sometimes ends up in my spam folder. I believe the main issue is down to the content of the email, which contains a few maybe not entirely code compliant sections and the embedded links that some filters pick up on. This could be mitigated if there could either be more options for customising the underlying template, e.g. allow the links to just be plain old links not embedded HTML links, or an option for entirely plain text emails.