Since some people seem determined on the question, clarification on what affects from the Aegis need to penetrate to function needs to be made.
It has already been clarified that the Aegis must penetrate (and at least match their Might in effect) to prevent beings of Might from entering.
Does it require to penetrate to prevent external spells?
Does it need to penetrate the magic resistance of beings within its area to give them a penalty on their spell casting/powers?
Note, that spells don't have MR. In its place, they have levels and magnitudes - whose interworking with Vim spells targeting them are described in the guidelines or descriptions of these Vim spells.
This is more complex.
But applying the MR of the being implies, that the being, not just its working of magic/power, is affected by the Aegis.
Hence the Aegis should also warp any person, that was not involved in its casting or later invited by token into it. Given the typical level of an Aegis, this means (ArM5 p.168 Powerful Magical Effects) one more significant source of warping to covenant staff and guests: 5 per year are typical. These accumulating warping points better be spelled out somewhere. I have not yet found the place where this occurs.
Furthermore we know by (ArM5 p.168):
Wards are active mystical effects as long as they are protecting someone. Two notable exceptions are Parma Magica and the Aegis of the Hearth, which are based on the same breakthrough by Bonisagus.
So those protected by an Aegis are explicitly not warped. It those penalized by the Aegis can resist, are hence directly affected and should typically be warped, we might need a similar statement for them.
Summing it up: to make beings use their MR against the penalty on their spell casting/powers, we need clear rules about whether the Aegis warps them.
Summarizing, and adding a first point:
Let me say just add that I think that the Aegis could be significantly streamlined, without fundamentally altering anything on which there is currently clarity or agreement. It really has two effects, that should be treated as simply and consistently as possible across all targets they apply to:
By the current rules a being is "foreign" unless it actively took part in the Aegis, or was invited and the invitation was not revoked. A magic item is "foreign" unless it was created within the Aegis, or the Aegis was created "around" it. This can create some confusion (e.g. how is a golem treated?) and some inconsistencies. For example, a hedge wizard who's stealthily brought inside the Boundary at the time the Aegis is cast sees his powers curtailed, an enchanted device does not. I think it would be significantly more streamlined to just say:
More streamlined, and far less useful.
Imagine a covenant with a large number of enchanted items used in many places in the covenant. (Magical lights, magical heating, magic oven, etc. etc.) These can't be actively included in the casting because they are stuck where they are, and nobody will want to go around issuing "invitations" to them all after every casting of the Aegis - especially not to the magical lamps hanging 20 feet up from the floor.
Nothing says they can't be included in the casting even if they stay where they are.
For example, naming them in the casting should be sufficient (I edited after your comment to make it explicit).
On the other hand, allowing an item that the casters do not even know about to be ok just because it's hidden somewhere in the Boundary is subject to abuse and should be avoided.
Something else for the Aegis, interaction with Supernatural Abilities and non-Hermetic spell casters.
Since Supernatural Abilities have a "Casting Total" and are treated as spells for calculating Penetration, does the Aegis reduce their "Casting Total" by half its level for foreign beings within the Aegis. While for some Supernatural Abilities it might seem silly (Second Sight, Sense Holiness & Unholiness), there are many magic traditions whose "spells" are actually Supernatural Abilities. My own groups interpretation is that it affects all non-"passive sensory" Supernatural Abilities, which means Second Sight and Sense Holiness & Unholiness are generally unaffected (except traditions which require actually casting them like Folk Witches), while Divination is affected. An official answer on if it affects all, some, or no Supernatural Abilities would provide clarity for groups who have not made a group ruling.
Looking at the affect on non-Hermetics the text is very vague, since it specifically talks about applying to Magi casting spells with no clear indication that it even works on non-Hermetic spell casters. While I have always seen it played as applying to non-Hermetics, that is not the way the text is written and is based on assumption. The specific mentions of "Magi casting spells" (along with much of the other text) has been copy/pasted since 3rd Edition. 2nd Edition is actually clearer since it only talks about spells cast by those who did not participate in the Aegis, with no mention of their form of magic.
Finally the effect on the Penetration of Magical Creatures should be clearly stated as affecting their Magic Powers, while any effect on spell and Supernatural Ability use should be brought in line with the final decision on the Aegis effect on said.
While most likely to out there for errata, there might even be an affect on Might beings ability to regenerate Might Points by resting within the Aura based on how the Aegis is defined. For example if it works by forming a barrier which interferes with non-natives ability to draw in fluid vis to work magic with, then it should also affect their ability to recover Might Points by resting in the Aura.
I've moved this to its own thread, because I suspect that it will not be errata, strictly speaking. However, Aegis of the Hearth is probably the only single spell that would merit further clarification in some form, so we should have the discussion.
As currently defined, it would seem that if an aegis is cast using a fence as a boundary then a mundane with a horse that pulls down the fence would destroy the aegis. Is this intentional or simply something that hasn't been considered?
There is actually another thread going on right now about a huge part of this. I'm pretty sure that thread is what propelled these questions to where you found them. Look here:
https://forum.atlas-games.com/t/magic-resistance-mastery-and-the-aegis-of-the-hearth/170011/48
Not quite. See LoH p.123 box A Note on Boundary Effects. So, a fence pulled down or moved does not affect an Aegis any more, once it is cast. Making the Aegis a type 1 spell, this is of course reinforced.
I think that silveroak means, with "current definition", the current proposal by David Chart (that invalidates the LoH insert): a Boundary ceases to exist for magical purposes when it is moved.
SInce the Aegis is obviously of type 2 (it affects even creatures that enter the Aegis after casting, and only for as long as they stay in the Aegis), this would mean that an Aegis could be wiped out by a "sufficient" change in the Boundary demarcating it - a change that could be enacted by very mundane means.
Oh dear! See:
To take part in a discussion about rules changes, one needs to keep track of it. Right?
Hmm ... I think that part has been removed in the last iteration, but I would bet more on you being right than on me. I am definitely starting to lose track of the current version of things 
Wards and Aegis all being of first type was likely removed in the last iteration, because it was needed for the discussion, but doesn't really belong to the rules about Containers.
@David_Chart: For clarity, it should appear directly in the description of Magical Wards (ArM5 p.114) and Aegis of the Hearth (ArM5 p.161f) instead.
The statement was never withdrawn, though.
Given that it is not included in the latest definition posted 1 day ago, the general impression is that it has been withdrawn even if it was not done so in whatever formal format you seem to believe is required.
Yes, I saw it. I will take it into account.
That sounds like a good reason not to use a fence/piece of string/chalk line on the ground as the Boundary for your Aegis.
But maybe that isn't how it would work.
It wasn't. However, it is up for discussion.
Wards on container targets are an odd case, because in order to work as they should, they have to affect things that are not in the target. Full logical consistency would say that Hermetic magic cannot do wards, but that's obviously the wrong way to go. Thus, we end up with hand-wavey explanations about affecting the space within the target, which tends to push towards type 1. However, you can be equally hand-wavey about type 2, so we could allow wards to be either type. Then you could ward a ship. Aegis is a special case, so it could be specified to be type 1, and thus unaffected by the disappearance of the Boundary.
Right now, these are all options. (And yes, this part would go in wards or Aegis.)
I have two issues with "it affects the space"- 1) if it affects the space then it should affect the space, not have to penetrate a tertiary target of something that enters the space. 2) the concept of space as a thing is a 20th century concept, and bringing it in to hand wave a problem just, from my perspective, makes a bigger problem.