Managed to get round the problem.

Thanks to your tip, I got the Sender to add their address
into a field in Excel and then copied it into the subject
line.


ActiveWorkbook.SendMail ("Payroll@marks-and-
spencer.com"), ("Debts From Sto ") & Sheets
("Debts").Range("D5").Value
MsgBox "Your details has been sent.",
vbInformation, "Debts"
Application.ActiveWorkbook.Close savechanges:=False

Where D5 is the Senders details.