Mail servers shouldn't have any problems at all, since they are not supposed to parse message headers and especially body. The only thing that matters is that the encodings should be supersets of ASCII – which both ISO-8859 and UTF-8 are... Besides, many messages still are 7-bit encoded (either Quoted-Printable or Base64) before sending, just in case it has to travel through a non-8-bit-clean server (even though all mail servers have been 8-bit-clean in the past 10 years).
Mail clients... honestly, I haven't seen any that lack support for UTF-8 for many years. (Perhaps somebody else has?)