I totally agree, I'm trying to save emails but there are a lot of people who don't reply after the job is fixed due to busy roles themselves. They usually thank me on passing or buy me a coffee haha.
Any idea how I can document everything ?
This is the line i'd be going down - documenting and logging everything.
Ultimately you'll need some kind of system to log problems, log what you did to resolve them, and how long it took. Ideally you'd also need a word document. a quick 1/2 page one to act as a feedback form. Ask what was done well, and ask what could be done to improve the service they receive. Ask them to print it off and fill in anonymously to get ultimately honest answers
The system we use at work is called Easy CMDB
http://www.easycmdb.com/Pricing.php Its a one off licensing fee and 250 CIs for the small business fee should be up to your requirements. You can create an incident for every specifc call, attach a document to that incident with the feedback form in it, it will automatically log times and dates for you etc..This is the sort of thing you need.
If the idea of paying sounds crap - then think about how you can improve the process of dealing with support calls with the system you've already got. In every IT job i've ever done, the logging of the notes about the problem has been done with the user on the phone. Get yourself a headset and type as they talk - that way its logged each and every time. Look into spiceworks and see if you can get it to create a ticket if it receives an email from a certain address. Our easy cmdb can do that, user sends an e-mail, automatically gets an email response with a ticket (we call them incident numbers, ITIL crap and all that) and you get something on your system with the entire body of their email in it.
Then all you need to do is keep on top of updating it when you're done.
That way when somebody else who isn't the person you ultimately dealt with gets funny, you have evidence to back up what you've been doing and how well.