This error occurs when using SMTP settings and there's no email address set up in the customer record.

'System.InvalidOperationException: A recipient must be specified.
at System.Net.Mail.SmtpClient.Send(MailMessage message)
at Sage.Email.SMTP.SmtpMailer.SendMail(Email email  SendMailOption whenToSend).'

To resolve this, check the correct email default then add this to their record.

1. Go to Settings then select Email Defaults. 
2. Select the relevant tab for the document you're trying to email. For example, Invoice.
3. Note which email is in the To: field. For example, Customer Record Email 1. 

This confirms which email field to check in the customer record. 

4. Go to Customers and double-click the relevant customer record.
5. In the Details tab, enter the email address in the 'Email Settings & Addresses' section. Enter in the correct Email field you noted in step 3.
6. Press Save.