Re: Cough... maybe we'll use letters and stamps next time.
As someone who covers first line support on a helpdesk as part of my job, I can confirm that email is not 99.99% reliable.
For example there was that time office364 decided that the provider we used to host the images for email signatures was bad and our clients started contacting us saying their internal emails were going to the spam folder... Followed by asking why we weren't replying, because Microsoft was totally blocking our replies for the same reason without even sending them to spam.
We have had many people email us, we reply, then 2 weeks later they email in asking if we got their email because they haven't had a reply. We tell users to phone us if they haven't had a reply from a human confirming we got their message within 45 minutes during office hours.
The company has received 2 formal complaints from users against the helpdesk staff in the 2 years I have been working there. One was against me for not replying to someone. The ticket system recorded my replies, the logs showed they had gone to spam...