Hello

Do you have errors in the logs?
Did you disconnect and reconnect after the change and clear the cache? you never know...

Dear
Now I have done all as you mentioned
but
when I comeback to the Mailing setting as you mentioned or login by other computer (never login into the projeqtor
the 4 items
- email of administrator
- 'from' email when sending mails
- 'reply to' email when sending mails
- smtp port
are remained old email setting
when I tried to replace with the new email, and click the "save" button, I got the message "No change to save on parameters"
There is a Log file
2023-12-10 10:59:03.182 ===== TRACE ===== DISCONNECTED USER 'admin'
2023-12-10 10:59:07.489 ===== TRACE ===== NEW CONNECTED USER 'admin'
2023-12-10 11:00:15.324 ** ERROR ** [V11.0.2] Error sending mail
2023-12-10 11:00:15.325 ** ERROR ** [V11.0.2] SMTP Server : mail.xxxx.com.vn
2023-12-10 11:00:15.325 ** ERROR ** [V11.0.2] SMTP Port : 25
2023-12-10 11:00:15.325 ** ERROR ** [V11.0.2] Mail stored in Database : #255
2023-12-10 11:00:15.325 ** ERROR ** [V11.0.2] PHPMail error : SMTP Error: data not accepted.SMTP server error: DATA END command failed Detail: Sender address rejected as spoofed.
SMTP code: 553 Additional SMTP info: 5.1.8
2023-12-10 11:00:15.325 ** ERROR ** [V11.0.2] PHPMail debug :

The email you use to send emails is rejected by your email server

Sender address rejected as spoofed.

Maybe you may configure your email server to allow this remote access.

If I user first admin email to send email, everything is OK;
I could not use the new email for sendinf the email.

It is an issue on your email server.
You must use existing and valid email address that your email server will not reject.

I tried this new email account; Works perfectly because of the same mail server system.
The problem occurred when I changed the admin's email from my personal email to another email (in this case projeqtor@);
Projeqtor saves all but logout and login or login from another computer, it's the same
Showing old 4 information:
email of administrator (old nam.nh@; new projeqtor@),
'from' email when sending mails (old nam.nh@; new projeqtor@),
'reply to' email when sending mails (old nam.nh@; new projeqtor@),
smtp port. (25 old; new 587)
and use it when sending test emails;

It seems you activated option "send email as current user"
You mail server rejects this : you connect to the smtp server with one email and try to send the email from another address.
Some server configurations will reject this unles the login account is expliciutely configured to allow sending for some other addresses
You should disable the option in global parameters