NSA325 log emails generate error

This is the mail system at host forwarder.cmp.livemail.co.uk.

I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below. 

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                    The mail system

 <[my email address]>: host mx.avasin.plus.net[212.159.8.200] said: 552

    Invalid 7bit DATA (in reply to end of DATA command)

________________________________________________

I take this to mean that the NSA325 is generating its emails in a format that may not conform to the standards expected by this particular email server (which belongs to Fasthosts, where my personal domain is registered but where emails are auto-forwarded to my IMAP account with my ISP, Plusnet). In short, I think the SMTP server is being less fussy than this intermediate delivery server, which is why this is showing up as a bounced email rather than simply a failure to send.  Because the emails are configured to appear to come from my personal email account, that is where the delivery failure message is being sent back to.

Can anyone offer an opinion as to (a) the accuracy of my speculation and (b) any possible means of correcting the error?  I can't see any way of changing the settings on the NSA325 to alter the format of the emails, other than choosing between HTML and plain text - both of which I have tried.

#NAS_Feb_2020