@Woogeroo Welcome to Vivaldi!
As your profile is new new, you might not have access to Webmail just yet. Access is now based on a reputation system.
Once logged into the Forum, go to this page. If you do not see a link to Webmail on the right side of the page, you do not have access yet. If the link is present, then you have access.
I just tested this from Vivaldi to Yahoo. The e-mail was not delivered, nor is it in Bulk/Spam, so Yahoo may be having issues at this time. On my mobile, I noticed the polling of the IMAP folders is rather slow.