The underlying Java library for generating Excel reports isn't great on memory usage, if you are generating too large of an excel file it can crash Mango. I would suggest you try running that report over a very small time range to see if that helps.
You can use a maintenance event, it can be either bound to a data source and all its points or just a subset of points. There is an option to schedule it or toggle it manually.
I see how that message can be misleading. You will need to look in your ma.log file to see what error you are getting.
If you go to system settings --> email settings you can confirm your SMTP server setting and test it from there. Then have a look at your ma.log file to see if it is an authentication failure or host failer etc.
Note that it will send a test email to the email address of the user logged in. So you will need to make sure your user has an email address.
@ramhuzaini Were you able to connect to OPC servers on local machine? I am exploring mango and just can't see OPC servers installed locally on my PC. The server dropdown remains empty, which I assume should get filled with all registered OPC servers. Will appreciate any tips on how to correctly configure an OPC DA source in mango.