SMTD-360 — Cannot write mail body line
Mail body line is longer than output buffer size. The description text specifies whether it was a regular body line or it was a SMTP/MIME multiline header. In the former case, the mail violates RFC and can never be processed. In the latter case, you can increase buffer size setting (smtp-proxy.client-conn.recv-bufsize, pop3-proxy.server-conn.recv-bufsize etc.).
While the mail is checked on input already, this error must be caused by mail processing (e.g. HTML filtering), or you have inconsistent input and output buffer sizes.
Mail forwarding/retrieval is refused.