Able to send out email, but can't receive email sent from outside

I host some websites and their email accounts, and installed ISPConfig3 at Ubuntu 14.04. Able to send out email from these email accounts, but can't receive email sent to them from outside. the SMTP, POP3, IMAP ports are set up at ISPConfig3, at the modem correctly.

why can't receive emails from outside?

 

the ispconfig3 is upgraded to 3.2.4 version at the moment, let me try to upgrade to Ubuntu 16.04 above to see how it will go. the latest ispconfig3 manual i have is version 3.1. should i have a copy of new version of ispconfig3 manual to refer to in this case? thanks.

 

should upgrade OS earlier, as ispconfig3 upgraded already. when both compatible with each, some of the issues should go away. thanks.

 

Thanks. I alone set up and configured this ispconfig3 system and hosted a few Drupal websites under Ubuntu 10.04 in less than 2 months in late 2010 for some IT business plans (and I was "promoted" to a Senior member in this forum. as I have other IT tasks to do and my goal wasn't for website system development, so since then I almost didn't involve with any of this forum activity, and I was "downgraded" to a lower status member, my understanding is that this is how the forum management functions here, which is understandable), then upgraded to OS to Ubuntu 12.04, and hosted these systems at my own home office for many years, at a German data-center for one years (from 2012 to 2013), then migrated to AWS Cloud Computing platform (I made a lot of effort to convert the VM format of this ispconfig3 system to be compatible with that in the AWS Cloud Computing environment in the very low computer infrastructure level) for a few years and during this period upgraded the OS to 14.04, and that's it. I haven't actively upgraded this web hosting system since migrating to AWS platform (I started to involve with AWS platform system in 2013, after I finished one year hosting at the German data-center, I wanted to find another solution to host these systems and other more important systems I was developing at that period. When I knew AWS platform was a better solution for my applications, I spent some time to pick up the AWS Cloud Computing environment, just reading the well-documented documentation and I found this was the most efficient way to grasp AWS Cloud Computing system. At that period I hosted those systems back to my home office LAN environment, and a couple of years later I migrated them to AWS platform for a few years until now. The purpose I learned about AWS Cloud Computing system is to host my own systems there at some stage, I know a lot of AWS Cloud Computing environments, but I only used a few AWS services for hosting my systems there. So far I think I spent a total of a few months, maybe 6 months or so on the AWS Cloud Computing systems/platform, and I can apply the other unused AWS services whenever I want to and there is a need for my systems hosted there. I don't like some people who learn AWS Cloud Computing systems to prepare the Certification exams, in order to look for a job. I have already had excellent education background/qualifications at the ICT fields, so I don't need to have this kind of extra certifications, and I consider the education/qualifications I have already earned are more difficult to be earned than this kind of certifications. I think I can earn this kind of certifications without too much problem, but there is no need for me to do so, while I just want to be able to apply them to host my own developed systems there at some stage), as these systems aren't important to me (in so many years I spent less than 6 months in total on the ispconfig3 system and these hosted websites, and the last time I worked on these systems was at a few years ago, but I think I have enough knowledge, skills and experiences on these systems to keep carrying on). Now I have another plan, so I'm looking into some new features of this ispconfig3 system again, and everything must be a lot easier than that many years ago when I started to set up and configure this system. By the way, before then I worked as ICT professional in New Zealand for over 10 years already, and I worked as ICT professional for a few years before I immigrated to NZ. Sorry please no worry, I can get all these figured out by myself alone via the research from the Internet, just haven't worked with ISPConfig3 for a few years, so I ask for the "email" question here (all in all, in so many years, all the above tasks have been always implemented by myself alone independently, without anyone else help, as I don't need others' help, except of asking ISPConfig3 people a few questions, never asking AWS team questions, as referring to the documentation of AWS services is better than asking them questions). Email thing is a bit tricky, as how well it works is involved with not only how well the ispconfig3 system and OS server setup, but also whether there are any issues at the ISP, domains registrar and hosting infrastructure sides also. Anyway thanks a lot.