What I'm saying is that mail is not reliable.
It's here quite possible that...
...mail messages can be mischaracterized as spam, either individually, or more broadly based on a filter that blocks all messages from the originating mail server; from me.com. A spam-filtering service can add a transient rule due to an outflow spam from other users of me.com, for instance. Or what the filter classifies as spam.
...the intended recipients can be using overzealous or buggy or misconfigured spam filtering, or can be using local mail scripting rules that are operating entirely as coded with arriving mail but not necessarily as the author of the rule had intended.
...the message contents might include some detail—phrasing, a string in the message or data within an attached file, even a specific filename—that is triggering spam filtering, in conjunction with the source address, or mail message route, or other detail.
The intended recipients can log a trouble report with their mail server provider. That report may or may not be successful. Transient spam-filtering rules can and do appear and disappear based on all manner of criteria.
Again, "The internet is held together with popsicle sticks and glue" — @jessfraz
Mail is not now and never has been entirely reliable, and the tsunami of spam is not helping.