You can do it either way:
Configure 123-reg dns settings:
point A records for www.domain.tld and domain.tld to your server
point an A record (e.g. mail.domain.tld) to IP of remote mailserver
Make sure mx record for domain.tld points to mail.domain.tld
Leave namservers alone.
Or
Set things ip in plesk similar to above and change nameserves in 123reg control panel for domain to be yours.
When email is hosted elsewhere, you need to switch off the mail service for the domain, as otherwise any email generated by the plesk server will be directed to the plesk server -- mx records are not consulted.
Configure 123-reg dns settings:
point A records for www.domain.tld and domain.tld to your server
point an A record (e.g. mail.domain.tld) to IP of remote mailserver
Make sure mx record for domain.tld points to mail.domain.tld
Leave namservers alone.
Or
Set things ip in plesk similar to above and change nameserves in 123reg control panel for domain to be yours.
When email is hosted elsewhere, you need to switch off the mail service for the domain, as otherwise any email generated by the plesk server will be directed to the plesk server -- mx records are not consulted.