Email server cannot be accessed or slow access from some computers

I am not sure if this is the right place to ask this question. I am not sure if this is technically a Mail-In-A-Box issue or a different issue altogether.

I have an installation of MIAB and it was running fine for almost a month without issues. Two days back, my client called and told me their emails are not working. Naturally I checked the admin panel, there were no issues highlighted, I restarted the server and still it did not work for them. But it was working fine from my computer.

It would fail to load or take too much time to load at different laptops. Trying in different browsers also did not yield any different result.

Initially I thought it was a DNS issue, so i asked them to hardcode the ip in their hosts file, but it did not help. I was able to ping the server, so their network is able to locate the MIAB server. I also asked them to use openDNS and public DNS servers, no progress.

Any idea why this is happening/how I should proceed trying to troubleshoot. Anyone who has encountered this issue before?

By the way, I live in India and my client is based in Dubai if it helps in any way. And my server is hosted on Digital Ocean’s Frankfurt datacenter.