Very little. "Mail delivered" means that LISTSERV created one mail message for a single recipient, and passed it on to the mail system. "Mail posted via SMTP" means that LISTSERV created a bulk SMTP transaction for multiple recipients and passed it on to the mail system. In the earlier days of LISTSERV, it was helpful to see the difference in the logs because "Mail delivered" was much slower. When reading the logs, it was your cue that it was normal to see a slowdown in the time stamps, and that perhaps you should reconfigure your lists to allow the use of bulk SMTP. Today it is unlikely that the difference is that big, especially with embedded mail-merge. But this still corresponds to two different delivery processes in the code that it is sometimes useful to distinguish for troubleshooting purposes. For practical purposes, the result is the same.