Not sure how to resolve current problems - have posted previously on both issues, but no response from anyone.
Problem A is that the "Warning: the Admin username is not a valid WordPress login. Postie may reject emails if this is not corrected." warning appeared several days ago, and there doesn't seem to be any way of resolving it. One other posting on this forum suggested changing the Postie Settings>User>Default User setting, but this appears to be only available as "Admin". What also foxes me is that this warning only appeared after an upgrade to the latest version of Postie.
Problem B is to do with graphic attachments. If an e-mail sent to Postie has attached graphics (as opposed to graphics embedded in the HTML of the e-mail) then sometimes (but not always), instead of the graphic appearing in the post, the graphic placeholder appears with a string of text (actually the file name of the attachment that was sent). Looking at that graphic in the image gallery shows it as having "greek" text and no graphic content, although if the file is opened for editing, the correct graphic will appear. The only solution I have found so far is to delete the "corrupted" file and upload it manually into the gallery, then insert it into the post. The really frustrating thnig about this problem is that it's intemittent - sometimes sending via Postie works perfectly, sometimes it doesn't.