If you are able to access your website, say www.domainname.com from your browser and not able to access webmail at http://www.domainname.com/webmail from your browser, then your ISP may be blocking the webmail ports, 2095/2096. In that case, you can access the webmail at http://webmail.domainname.com with your email account's log in credentials as this will be connecting to the apache port of the server.

If you are using any email client, say Outlook and you are able to receive emails and not able to send emails, then you can try changing the outgoing port to 9925 or 465 (if using secure SSL for SMTP) and make sure "my outgoing server requires authentication" is checked: use same settings as incoming and try taking Outlook again.