EMAIL DELIVERY ERROR: WP Mail SMTP plugin logged this error during the last time

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Hi. I have searched for this issue but decided to open a new thread because my case is a little different.
I got a messsge displayed at the top:

EMAIL DELIVERY ERROR: WP Mail SMTP plugin logged this error during the last time it tried to send an email:

Mailer: Other SMTP
PHPMailer was able to connect to SMTP server but failed while trying to send an email.
Please review your WP Mail SMTP settings in plugin admin area. Consider running an email test after fixing it.

I have tried sending test mail and it successfully sent each time I tried. After the test successfully executed, the message dissappear for a while and eventually will reappear.
I am not sure what is the issue because the mail is working fine.

I am using SMTP btw.