This is a question that I have been asked many times via email and sometimes even phone!
Situation is this: you wake up in the morning and found your email hosting is down. You are not getting any emails, either they are bouncing back or going to a blackhole.
What should you do in such a situation?
Email service is much like a chain – if any part is broken, you won’t get things working (apart from lots of frustration!). Since it’s like chain, one must follow step by step to find the problem.

So at this point a good question can be – where does the chain start?

Email hosting consists of 3 parts:

1. Domain Registry record
2. DNS Hosting
3. Email Hosting

Domain Registry record:

Start by checking the domain registration. You can do that using any whois service like who.is, drwhois.com etc. Make sure your domain isn’t expired. If it is, quickly visit your domain registrar control panel and renew it. Most smtp servers keep re-trying to deliver email for around 48hours, so if your domain is expired, you should renew it quickly so that future attempts at delivery will work correctly.

DNS Hosting:

If domain registry is OK, then proceed with checking things at DNS hosting.
DNS servers play an important role in email delivery by giving your MX record to sender’s server which is like an “address” of email host. So you can try sending a DNS query to dns servers for MX records.
You can use online nslookup tools like Zoneedit’s nslookup or network tools nslookup or if you want to try to lookup from your system, use “nslookup” command in Windows DOS (as shown here) and “dig” in Linux (shown here). If server doesn’t returns MX records, then try looking up for “SOA” on that server for your domain. You will probably find no SOA records which “technically” means that server isn’t hosting dns records for your domain. In real world it can be billing issues with host or accidential removal of domain. If you don’t see any SOA records, immediately contact web host via phone. If you are using any decent web host, you can expect things to be up within a couple of minutes. Otherwise I would recommend using any free dns host like editdns or zoneedit. Just signup for a free account with them, add your domain, and then DNS records (MX and A records) followed by change in name servers of your domain.

Email Hosting:

If your DNS server is returning MX records, you can proceed to the next part of chain – email hosting. If your MX records are OK and you are still not getting emails, your of email is likely bouncing and the bounced email will include the “cause” of the bounce back. It can be email account doesn’t exist or server is not accepting email. In most such cases, you would have to contact your email host for help and it can take a day or so to sort out the issue. The best thing you can do in such time is set up a temporary redirection – based email host to prevent bounce back.
You can use free email forwarders like zonedit or editdns to get all incoming emails forwarded to your personal gmail/yahoo account.
To setup forwarding – signup for a free account with provider, add your domain and setup email forwarding for all incoming emails.Then query the dns server of that host for MX records of your domain and set those MX records at existing dns hosting (to prevent hassle of shifting dns hosting).