Right now we’re sending mail via Amazon SES, and rather than spoof the “from address” (which is what we were doing before) we are setting your address in the reply-to header (Account Settings >> Routine Emails). So right now any mail sent via SES will pass SPF and DKIM checks, this should be more than enough to result in very high delivery rates. Also when the client hits reply it will come back to your email.
The recommendation in the post you mentioned was relevant before the transition to SES in October. What we are doing now by default is much better as we’re sending from a @quickfile.co.uk email but setting a customer reply address. So any reverse DNS check will pass and you’re still getting the client replies.
It should be noted that nothing you do will guarantee 100% deliverability. Email and SMTP is a complex subject and there are many rules implemented to combat spam, some of which will trigger false positives and most of which are not publicised and are based on proprietary algorithms. Just having an email with a certain word or string of words is enough to route it to junk.
For those cases where the emails are going to spam, your client’s email software may provide a reason but it’s likely to be quite general.
As for the SMTP error you’re seeing, a Google search indicates that it’s likely to be Hotmail blocking the attempt. The post there indicates how it can be white-listed.
Try the above and let me know if that works.