I can now confirm that email was lost during Spamgourmet outage. I have automated monitoring for SG email forwarding using 30 minute interval. Quite big batch of email bounced back and never went trough. Having backup server (even just a queue server) would have solved this issue.
As I have said here and in many other forums, with SMTP there are two inherit problems when servers aren't working or performing as expected.
Capacity problems
If server is too slow or all SMTP sessions are all being used or any (4XX) error code is returned. Then messages start do disappear in random! This has been happening with SG several times. At one point I think it was more like a chronic problem. Mail got lost all the time, but just very small percentage from hosts that had only a few retries and had short queue expire time. Servers with more retries and longer queue times did get mail delivered. So problem wasn't obvious for everyone.
First clue to users about this problem is that delivering email start to take (at times) longer than usual. It's clear indication that this problem exists. So if you send email to your self as a test, and it takes more than 5 minutes to get delivered. Of course you can do more advanced analysis on SMTP level, but I'm sure that 99% of users aren't going to do that anyway.
Down time
If server is down for too long, messages start to disappear but not so randomly. Disappearing order depends from sending servers queue timeout. Which can be nowadays even something like 4 hours. So if mail server is down for 4 hours or more some email starts to get lost. For that reason backup server is really essential requirement.
Back in old good days severs often kept emails in queue for at least one week, but that's pretty rare now AFAIK. First warning about mail not getting delivered was given in 24 hours but server still kept retrying. I know that some servers now give out warning just in 15 minutes.
In 2008 I wrote:
When all SMTP connections to server are used, process of email delivery starts to become random event. Meaning that there is change that email gets through immediately and there is also slight change that email doesn't get ever delivered.
It's just like tossing a dice. You got 10 tosses. When you toss dice and get one, email is delivered. There is also change that you don't get one at all with those 10 tosses you have. In that case email is lost.
So email delay is not about emails being in simple FIFO queue, getting just delivered late.
And that's why this situation is unbearable. IMHO
Conclusion
My honest recommendation is that you shouldn't use SG for anything important. If email gets lost, you shouldn't be upset.
I have also run many free web services and created freeware applications. Nothing is more demotivating than people bitching about free service/software they're using. Trust me, I know.
If SG isn't enough reliable for you, then use another alternative. - Thanks.
I can also remind that SG is open source, so you can install your own instance at anytime. Just remember to Administer it better. If you just think you can.
Best regards,
Sami Lehtinen
www.sami-lehtinen.net
Edit: A few typos fixed.