CHECKIN, CHECKOUT and HOLD digest notices are not generated "all at once", but we generate a notice with the first action and then keep amending it until the notice is sent. This creates a number of problems:

* We cannot make sure to include current/updated information in the notice, this might lead to hold pick-up notices be sent out for already picked up holds for example.

* We limit what we can do with TT and need to rely on repetition markers ----. This is non-standard and we often had them deleted and notices broken in support.

...

From bug 15278:

> (In reply to Martin Renvoize from comment #23)
> 
> > As for the --- digest thing, I feel like the logic for that one is backwards
> > and strange.. we should be generating the notice all in one hit at a later
> > point in time rather than queuing a notice and then munging extra stuff into
> > it after the fact at various times.
> 
> The --- digest thing also doesn't make much sense to me. 
>  With putting it together at one point in time, would that mean that say for
> hold notices when notices send every hour that before that happened it would
> build and check for the patron any waiting holds that had been set to
> waiting since the last one? 
> 
> Or perhaps we could set some sort of flag when the hold is trapped that gets
> cleared when the notice is sent?

If we know the exact time something was checked in, checked out or set to waiting, we might be able to use this information to build the notice. But then maybe need to remember when the last notice was sent? The flag might be a more reliable idea and also would make it easy to resend some or all whenever needed.