Because I use my own CentOS system to install the aaPanel control panel and set up my own mail server, The mail server software name is Mail Server 4.3.
This is the information of the mail server:
Email Name MailBox space Type Status Operating
service@xxx.tw xxx 2GB Admin
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Note: The current mail server supports IMAP/POP3/SMTP/HTTP protocol. Download the HTTP-API documentation
POP service [ ServerAddress:mail.xxx.tw Port:110/995 ]
IMAP service [ ServerAddress:mail.xxx.tw Port:143/993 ]
SMTP service [ ServerAddress:mail.xxx.tw Port:25/465/587 ]
When I set up the mail server, Only such information:

Domain name xxx.tw
A record mail.xxx.tw
Of course, I have already set the A record in my domain management panel, I can also use Thunderbird to send and receive emails normally.
But I still can’t use OpenCart to transmit normally, Can anyone tell me how to solve it?
My environment:
OpenCart 2.3.0.2
OpenCart 3.0.3.2
Currently using OpenCart 3.0.3.2 as the test system.
Thank you!