Welcome to the community,
Could you please send me a PM (private message) with your Full Name, CID (customer identication number) and your mobile phone number, so I can take a look at your service?
Can you please include the email address your sending from and to?
If you don't want to provide any details that is fine, but there isn't much help I can provide.
From the error message you provided it sounds like you are sending to an invalid email address. I hope this helps.
Do you only use the Online Mailbox(Webmail) or do you also use a mail client on a computer or phone?
And if you do use a mail client do you get the same error?
If you could PM me a couple of example email address that you have problems with that would be great?
I mainly use a mail client (Outlook on a pc) and cannot send. So that is when I tried the Online Mailbox and received those errors.
I can type in any valid email address to yahoo, gmail etc and still get the error.