Why ? Random Tech Questions

ceagee went on a bit of a rant said
4

I have a tech question that I thought some of the more tech savvy members of the community could answer. I also thought I'm probably not the only one with a random tech question. Hence, a Q&A thread.
If it doesn't work out, it's all @cengland0's fault.

Q: I have an old yahoo email that is a made up word, but it could be a "real" name. There are no caps, no symbols, no spaces, just regular ordinary alphabet letters. It has served me well for many many years. Recently, when using it to check out on a site or fill out some sort of form for something, I got back an error message that said something to the effect that my email is "invalid" and could I please try again. Thinking I hit a wrong key or something I tried a couple more times. No go. And now this has happened several times at different sites. I end up screaming something like: "it is valid you morons" at my computer. Some sites suggest I leave out symbols and spaces, etc.. but there aren't any. This leads to more yelling.
What's going on? It still works most places. Why do some sites suddenly see my email as invalid ?
Thank you.

TL;DR Some websites don't like my old email.