My 2 cents:
Missing just one email indicates some sort of issue, missing two indicates more stongly there is some sort of issue.
Possible issues:
1 - There is simply to much work to reasonably monitor email to the level of completeness required (I’ve been in this situation, even though they were flagged for follow up, there simply wasn’t enough time to get to them in a timely fashion)
2 - You have a procedural issue that is allowing some emails to slip through the cracks
3 - It’s an aberration, it hasn’t happened in 20 years and unlikely to happen again
If the issue is either #2 or #3, the next step is the same:
1 - Accept complete responsibility (don’t counter with anything)
2 - Briefly explain your new bullet-proof procedure you’ve implemented to address the problem (e.g. “I’m now flagging anything requiring follow up and then flagging when complete, I review all open flagged items every morning”)
Nobody above you wants to hear excuses, just the actions that will eliminate the problem. Even if the issue was truly a one-off that hasn’t happened in 20 years and probably won’t for another 20 years, you can’t really explain reality and have others understand it, you can only show them you take the problem seriously and you have taken steps to correct it.