1. All web servers are potentially vulnerable to compromise.
2. Hosts often don't have an 'own up' policy when things go wrong. This is for anumber of reasons and can include anything from general embarrassment, loss of reputation/face/revenue from irate clients etc
3. You can write a server testing script that will also test the email server and schedule it to fire off at a date/time of your choosing or use a web interface to run the test (a bit like webmail). Alternatively you can use third party server monitoring script - a search will produce tons or click here for one which can be automated to run as a scheduled task from your own PC and will log all findings.
All email servers run by hosts will normally be 'throttled' during difficult times to prevent further load issues compounding the problem at hand and the fix(es) being implemented.
It is perfectly normal, especially where mass emailing is the issue to begin with. Failing to throttle the email servers just makes solving the problem far more difficult and also increases the risk to legitimate email traffic.
What I find worrying here is that Easily had similar issues within the last few months where a major penetration took out most of their resources and many sites were badly affected. Although it can and does happen now and then (even Microsoft have been well and truly hacked into several times) such a serious compromise being repeated in so short a term is hardly inspirational.
I wrote my own server monitoring script some time ago and it fires off from a very, very reliable soucr point to test all sites under my control. It has proven useful in testing for faults over time and has helped me decide who to use for large scale hosting services based on fault-free service delivery over long periods.
Easily have been good to me in the past and it is not my place to either defend them or condemn them The attack could have been direct at the server level or through a user account through a third party script with a nasty security bug in it.
We could speculate on the cause all day.
It's up to you how you want to proceed, but all web servers can fall foul - the safest web server is one that is never connected to the internet...