Allow saving a full log of SMTP communication to database

We have a "little" problem: Some mails are sent properly and arrive swiftly. Other kinds never. No error in watchdog table.

Could you add a debug feature to save all communication with SMTP server to database, for future reference? We want to know if there is some irregularity that might affect sending, and we need to know what we want to ask / request our SMTP provider. Full log of how we see what's happening would be extremely valuable for this.