One other option we have is to remove the reply-to address on your account, by default we send from mail@quickfile.co.uk but add your email as the reply-to address. Some email servers don’t like this. By removing it however it means that your client will not be able to reply directly to the email, however the delivery success rate will likely improve.
Failing that the other option is to setup your own SMTP email server details on your account. QuickFile fully supports custom email servers.