“Retainer” is one of those “slippery” words.
For instance, for you, it may mean “In addition to the tasks that are pre-defined, the client can and will contact me whenever they really need me, within an agreed monthly budget of x hours.”
For the client, that same agreement may sound more like “In addition to the tasks that are pre-defined, the expert will be CC-ed on six different email chains every week, and chime in with expert advice they think could help, within an agreed monthly budget of x hours.”
The key difference in interpretation here is the client doesn’t naturally include your reading of endless email chains as a part of what they pay you for.
In other words, if you spend 6 hours a week slogging through the back-and-forth emails, and then make four suggestions, one of which is accepted and takes you two hours to do, the only “value” the client will see is what they got in those 2 hours. You spent 10 hours this week, but they only appreciate the last 2.
Sooner or later, that slight, silent difference will grow teeth and bite you – possibly wrecking the relationship. This is not a good deal, for either of you.
To avoid that, address it in advance, and make sure to get a clear answer about whether it’s a good use of your time (and therefore your client’s money) for you to be included in the emails, or if there’s a more efficient way for you to stay in the loop.