I like my mailbox organised. And I like things to be automated. Fortunately, email systems support aliases for their users, so more than one email address reaches the same person. This allows for automatic filtering depending on which address the message was sent to.
What’s even better is that these systems can match a pattern to make generic aliases (e.g., user-REPLACEME@example.net for user firstname.lastname@example.org). This way, you can create valid email addresses on the fly, without having to tinker with anything (e.g., email@example.com for the gas company to contact the user).
Now, dash (-) is not the most common character used for that purpose. The plus character (+) is more commonly seen. Notably, but not alone, GMail supports it. If you have an account there, try sending an email to YOURUSERNAMEfirstname.lastname@example.org.
And this is where my problem is. Once again, I was happily filling in a form requesting my email address, put in an address with a + in it, and got it rejected because it “contain[ed] invalid characters.” It really annoys me that some people who call themselves professionals in IT-related fields do not seem to be able to understand a standard properly, if they have been looking for it, at least…