eM Client creates a random email address and tries to send it but the server refuses it because it it a fake address. See below…
1:09:04 AM [email protected] SMTP Sending messages: Recipient address ‘u+mnpwszb5njddo6dpj5euqrcsou2vs52vjjiwc4teezsd2ntdme4domrgmvwwc2lml52hs4dfhv3w64tlmzwg65zgmvwwc2lml52hs4dfl5uwiplgmu2wenlemyyc2n3ggy3c2ndbmjrs2ojxgi3c2mrumu2dgyrygnsdmyrtezud2nrrgzsdqobuge4tkojxmjrgeztcmyytcmdfhaygeyzqgrrggztfezut2mrqgiztanbsgyytqmrzgq2s4yrrmvrtknbtme2tozrqgnsdimzfgqygy33dmfwhezlnnf4c4zlnmfuwyjtmn5rv62lehvtueyrqjqzwo6tnljrhmwdzoyyxm2lbnitheplbojsxgjjugbzxkzdemvxgy2lonmxg4zluez2d2jjsie@localremix.email’ not accepted due to the following reason:
"RCPT Invalid mail address, invalid local name syntax
That is the actual address from the message you are replying to.
If you select a message and click reply, first the From address will be used. But if there is a different Reply-to address, that will be used instead.
You can see the message header by right-clicking in the original message body and choosing View Mail Header. Look for the From: and Reply-to: headers.