How can i access webmail without certificate error?

I assume your server is using a self-signed certificate.  Therefore it is not trusted by any computer outside your domain.

You have two options if you want to prevent the error message:
1. Buy a Certificate from a Trusted Certificate Authority and set this up on your server
2. Install the Self-Signed Certificate into your computers Trusted Certificates

Obviously number 2 would require you doing this for every computer that connects (or giving staff instructions on how to do this)

I prefer option 2. How can i do this?

Microsoft TechNet has a step by step guide at:  Installing a Self-Signed Certificate as a Trusted Root CA in Windows Vista

This guide is for Vista, but it is similar for other Windows Versions.

I do not have "Local Computer" in "Trusted Root Certification Authorities". The options that i have are "Registry" and "Smart Card'.

Do you have administrative rights on your local computer?

I followed the steps from Technet and i am still not able to access web mail without the certificate error. You can view my screenshots. Maybe i am missing something. I am also using Windows 8.1 .
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The certificate appears to be issued to yourdomain.lan

Is this the address you are accessing it from?  If your accessing it from over the internet I assume it is not.  Therefore the certificate should include 'Subject Alternate Name' for the different addresses the site can be accessed on.

If it only contains the .lan address and you are using a different address it will cause an error.

From the server "Certificate Authority" i downloaded the certificate. The date shown on the certificate is "Valid from 11/10/2013 to 29/8/1902. From the browser the certificate error shows "Valid from 11/10/2013 to 9/10/2023". Could this be the reason why i receive the message on web page " Continue to the website (not recommended)."
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These appear to be two different certificates.  One is a self-signed certificate for your CA, the other is a certificate issued by the CA for skygreece.xxxx

The dates being different is therefore not a concern - they are separate certificates.  However, the fact the CA cert is already expired is a concern.

Effectively if the CA certificate has expired then the computer will not trust it.  Therefore any certificates issued by this server also cannot be trusted.  I think you need to reissue/renew your CA certificate.

If your using Microsoft Certificate Services, instruction for this can be found here:  Renew a root certification authority

The CA cert does not show that is expired. The expiration date shows 9/10/2023. The other certificate shows a date of 29/8/1902. Are you referring to this certificate as expired? With a date of 1902?
I downloaded the Certificate Authority from a Linux server. How can i renew the certificate?