correo

In pronouncing “correo”, the stress is on the “e”, which seems to violate the rule of stressed syllables.  Isn't the “eo” a diphthong?  Compare this to “farmacia” where the stress is on the second “a”.

Which rule are you referring to? As I understand the rules, since it ends on a vowel, the second to last syllable (rre) gets the stress. The complete rule:

If a word ends with a vowel (a, e, i, o, u) or with an “s” or “n” following a vowel, the stress is on the penultimate syllable (the second-to-last syllable). In all other cases the stress falls on the last syllable. 

https://jakubmarian.com/stress-position-in-spanish-words/