When emailing statements directly from MYOB, you are currently limited by the NUMBER of characters permitted in the default email information that gets sent with the statement (also same with invoices).
Outlook does not automatically add the Signature default, no matter if you select to include in ALL reply emails, when sending emails created from witin MYOB.
Solution: Either to increase the number of characters permitted in the email default settings to allow messages to be included and/or outlook signatures including logos, or to better interact with Outlook (and other mail services) to attach the default signatures stored in Outlook (for example).
"Default Email Message Character Size Limit - Emailing Statements/Invoices"