SMTP error 550 failed to add recipient .........

[Chess]

Hi all,
Hope someone can help.
I am trying to send mail out and keep getting a 550 error. I have tried adding contacts to address book before sending mail  but that does not work. I have looked on the net and have seen advice on changing login details and .ini files. I am using roundcube in cpanel installed by my web host so not sure whether I have access to these files. Correct.....this is all new to me and whilst I appreciate all advice please keep any pointers as simple as possible. Thanks in advance

[SKaero]

You need to contact your host and ask them to fix the problem.

[Chess]

Thanks Skaero