Contacts deleted from "To" address line on send

I regularly send emails to a group of around 12 people. I copy and paste their addresses from a spreadsheet. These then show up in the “To” address bar. However, on sending, the last 2 or 3 names seem to be dropped from the list. I am not using Gmail, but collecting and sending mail via btinternet.com.   Please help

Hello Carolyn,
do you copy them all at once? Is there perhaps some typing error or missing symbol on the 10th/11th line?
Either way, I suggest either checking thse lines or, if this is a list you often use, made a Distribution list directly in the eM Client’s Contacts section. Using the name of this list will then put all the addresses in the To section.
Just go to contacts and create New > Distribution list.

Regards,
Olivia