Error Sending Email. Value Cannot be null. Parameter name: address

I am getting the above error when trying to send an email.

Full message is

System.ArgumentNullException: Value cannot be null. Parameter name: address at System.Net.Mail.MailAddress..ctor(String address, String displayName, Encoding displayNameEncoding) at Nop.Services.Messages.EmailSender.SendEmail(EmailAccount emailAccount, String subject, String body, String fromAddress, String fromName, String toAddress, String toName, String replyto, IEnumerable`1 bcc, IEnumerable`1 cc) at Nop.Services.Messages.QueuedMessagesSendTask.Execute()

However, when I use 'Send test email' it works.

I am getting this error when user tries to send an email response with active chat. That is if the operator is not present.

Thanks in advance for the help.

Prathima

(To) Email address is missing some point.

Thank you very much for the reply.

I have gone into the queued messages (administrator section) and when I open them they have both to and from email address set.

Prathima

What about to name? should be empty or something, NOT null

Both ToName and FromName have values too.

only fields that are not filled are CC and BCC fields.

I have noticed 'ReplyTo' field is null.