There is someone I know who uses a gmail address for work. The organisation she works for is quite small and they have been using gmail as their main contact email. She has to talk to people at universities and it seems that some school servers have been blocking her emails. Any suggestions on free tools that could be used to check if that particular gmail address has been blacklisted?
Do a web search on the terms "check email blacklist". you will find a number of sites that can check the various public blacklists.
Note that organizations and people may have their own black/spam lists as well, and unfortunately, the only way to check against them is to send something to them and have them check their mail logs.
It's quite possible to accidentally get blacklisted - it has happened to me. In fact, Google actually spam-listed mail from myself to myself! But there are generally ways to get removed from such lists if it's a genuine error.
Bjoke: A "Bully Joke". A Statement or action made with malicious intent - unless challenged. At which point it magically transforms into "I was just funnin'" or "What's the matter, can't take a joke?"