Mine's an iMac too, but the original model, 11 years old. I can use all sorts of accented and special characters and symbols, but after I post them online they transmogrify into rectangles or garbled text. Here's "sincere" with something that looks like the long s, circumflex on the i, tilde on the n, cidilla on the c, and grave or acute accents on each e. It looks fine in the text entry box before posting.

∫����r�

It looked fine in the text entry box before I pressed "preview." Then in the instant it turned. On the preview text, the long s had squared off into the integer symbol above followed by question marks with an "r" among them, while in the text entry box the long s has become ampersand, number sign, 4-digit numeral 8747, and semicolon, before the "????r?".