Problem adding MX record to domain settings

I transferred a domain from 123-reg.co.uk to SS on 13.3.20. The email address for this domain is still with 123-reg.co.uk and I have tried to add the two MX records they gave me to the domain settings in the domain. They are as follows:

However, I get a warning that this is "Not a valid host name".

123-reg.co.uk insist they are correct and this is the only information I need to be able to connect the email address to the domain.

Can anyone advise on what the issue may be?

Thanks,

Mark 

An MX record has three parts, the host name, the server, and the priority. The table you included has the last two values of each MX record. The host name is the host name to which you address the email.

For example if your domain is domain.com and your email addresses for that domain end in @domain.com, then the host name is @. 

If the email addresses for your domain domain.com end in @mail.domain.com then you would set the host name to the value mail.