Mail to us is never silently dropped. Either your mail makes it through, or you will get an error message back listing the reason for the rejection. (That's propably why you are here, reading this.)
We view email as a medium to exchange small, simple text messages. The following restrictions are in place to enforce no-fuzz, simple communications between you and us. The restrictions also help to weed out most spam.
Beyond these strictly enforced policies, you are most welcome to honour these email preferences which hold for the entire user base here.
Exceptions to the strict rules are possible. Contact us with plain text email or phone us. There are simple ways to send any MIME type, for example.
The mailserver you use (yours or your provider's) must have forward and reverse DNS entries, and they must match.
The mailserver you use (yours or your provider's) must not be listed on one of the zen.spamhaus.org DNS RBL lists. If it is, the error message will contain a URL indicating what the specific objection against your mailserver is and offer remedies.
Firstly, we do not decode HTML-encodings for security reasons. Even if we would, users would usually not be able to view colors, different fonts styles, or different font sizes and completely miss that urgent important part written in red.
Do not send us binary material of any form and contents in emails.
Please enclose smaller clear texts directly in the message. Don't send screen dumps to us if possible. In particular, we ask you to to copy error messages from, say, a GUI dialog box manually if cut'n'paste won't do, certainly if it's just three lines of text.
Do not send us Microsoft Word docs or other "Office" documents. We don't use Word or Microsoft Windows at all.
Rather unfortunately, multipart/mixed also applies to vcard attachments. We are actually not at all opposed to vcards per se. As long as you send it just once instead of always. Which you would. Hence no.