Well, of course it would be wonderful to prevent them altogether on lists, but ...

Anyway, in an Exchange (Office 365) environment, you should be able to use the Powershell cmdlet
  Set-RemoteDomain
to specify "AllowedOOFType" -- we have specified "none" on mail sent to the listserver. 
  https://technet.microsoft.com/en-us/library/Aa997857(v=EXCHG.150).aspx

However, I just tracked a case of someone being served off while out of town over the weekend of the Fourth, and was wondering if others had encountered similar.  I'll contact MS about this, but thought this group might have more direct experience.
Thanks,
--Joe

oh, fwiw:

PowerShell> get-remotedomain | ft -auto

Name                        DomainName          AllowedOOFType
----                              ----------                    --------------
Default                     *                                External      
UGA Listserv            listserv.uga.edu     None          
UGA Listserv CC      listserv.cc.uga.edu None          

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