Error: 501 Invalid base64 data

[glenncjblack]

I tried to do a test email transmission for 1 plain text email via my SMTP mail server, and received the error "501 Invalid base64 data". What does this mean? What do I do to correct the error?

 

[stanbusk]

Can you paste here the text of the delivery panel where you see that error? I need to know the context.