Undelivered Mail Returned to Sender

[toanle]

Hello all members,
I have one issues when i send email from roundcube to one gmail and i got email is: "Undelivered Mail Returned to Sender".
And when i try use php for send email, i get true but no email receive.
I just start using cwp so i have not more skills, So i need help from members.
Please help me.
Thanks all!

[Sandeep]

check the logs /var/log/maillog

[toanle]

Hello,
I checked but i have not this folder.
Also i can saw: "/var/log/mail" but have one file can not read. it is statistics.rpmsave.

How to i can check next step? Thanks for help.