An invalid character was found in the mail header

Email Send Error: An invalid character was found in the mail header: ','
How do I sort this out please?

You will need to remove the comma from the email address. Commas aren’t valid characters in email addresses.

Thank you Glen - I checked - the email address I tried to send it to is pmj@apaa.co.uk. I just used it to send a test message from the support page which arrived immediately. Can you think of somethink else please?

It’s the sender address not the recipient address. Go into Account Settings >> Routine Emails, scroll to the bottom and you can see a comma instead of a full stop in your default sender address.