I just did it. And didn't know whether it is correct. If there were any mistake, please correct me.
It is because someone accidentally deleted an information, hostname, in DNS Administration page.
This is single server. I would like to add my server hostname to DirectAdmin so that my server is ready to send and receive emails.
I went to Admin Tools --> DNS Admnistratrion -->Add Zone
Domain name = myhost.mydomain.com (this is my server name)
IP = xxx.xxx.xxx.xxx (my server IP)
Name server 1 = ns1.mydomain.com ( I have already added in DNS Administration with local data=no, local mail=no)
Name server 2 = ns2.mydomain.com ( I have already added in DNS Administration with local data=no, local mail=no)
Then, I clicked on Add button.
Then, I make sure that what I have done according to
https://help.directadmin.com/item.php?id=16
Now, both send and receive mail functions are OK. Anyway, not sure that these are all about what I should do.