The phonetic value of Portuguese /e/ varies somewhat,
and it sometimes tends to be opener (lower) than a cardinal /e/ and almost as
open as a cardinal /E/. To maintain the contrast, Portuguese
/E/ travels away from cardinal /E/, in the
direction of cardinal /a/, but without becoming a Portuguese
(By the way, Portuguese shares this phenomenon, that is, the second part of it, with Hungarian.)
Some samples to illustrate my point:
In estrela ©, I hear a very clear [e], which is certainly not [E], but in the word mesa © (also in this © sample), to my ears the vowel is more like [E], although it is officially supposed to be [e]. The same can be heard in the word tristeza ©. In the following example there should be a contrast between the /E/ in esta and the /e/ in tristeza ©, but I find it hard to hear this.
(Addition 18 February 2013: Now – after a lot more exposure to the language than I had received back in 2000, when I probably wrote this note – I do hear the difference.)
In this song sung by Camané, "recomeça" is in rhyming position with "aconteça", although in theory they have different vowels: recomeça © is from the verb recomeçar, and ar-verbs have open stressed vowels o and e. aconteça © on the other hand is from the verb acontecer, and the subjunctive used here is derived from the first person aconteço, which (unlike the third person acontece /3ko~tEs1/) has a closed vowel, so it's /3ko~tes3/. But in this example I hardly hear any difference at all. And I don't think the singer modifies his pronunciation to force the words to rhyme, because in that same song, redor © (with a half-open vowel) is in rhyming position with amor © (with a half-close vowel) and here the difference can be clearly heard.
Verbs ending in -er are also supposed to have the /e/ sound, but listen to
is yet another example, where the vowel seems to be almost [E], but is [e].
I also have an example, however, where the different sounds do contrast very clearly: um lírio aberto a ser na alvorada ©.
The nasalised /e~/, which is supposed to be the nasalised counterpart of /e/, not of /E/, also varies in the same way as /e/. Examples: mentira ©, desventura ©.