How do I configure DNS records for a mail server?

Assuming you are trying to handle mail for the domain itself, for addresses @synologyvietnam.vn, the “Name” of your MX record shouldn't be “mail”.

MX 紀錄. MX 紀錄，或稱郵件交換紀錄(Mail exchanger record，MX record)，用於標示郵件如何導引至目的地。請將MX 紀錄指向您的郵件伺服器，來指示網路要將郵件寄往何處。

MX 紀錄：用於指定代表網域名稱收取信件的伺服器。 · A 紀錄：用於將伺服器的主機名稱對應至其IPv4 位址。 · SPF 紀錄：用於列出可代表網域寄信的授權伺服器。

Mail exchanger record (MX record) indicates where emails should be routed to. You have to set up an MX record pointing to your mail server so that the Internet ...

An MX record or mail exchanger record tells which mail servers accept emails on behalf of a domain and where emails sent to your domain should be routed through ...

An MX record or mail exchanger record tells which mail servers accept emails on behalf of a domain and where emails sent to your domain should ...

MX 紀錄或稱郵件交換紀錄列出代表網域接收信件的郵件伺服器，指示信件如何透過SMTP (Simple Mail Transfer Protocol，簡單郵件傳輸協定) 寄送至您的網域。

I'd like to set up my Synology Mail Server with a dynamic IP. I'm not an IT professional and hope ou can help me here out with this ...

The A record should point to the IP address of the mail server and the primary MX record should point to the name you used.