Hello. Does anyone have any idea why sqwebmail is UTF-8/I18N encoding the subject line, just because I put a single = sign in it?
Because, as you note, = is used as part of I18N encoding itself. Therefore, if = is explicitly entered into the subject line, and it's followed by a ?, it will get mis-interpreted as the beginning of an I18N string (with disasterous results). To prevent that, the = character is also encoded.
Although in this particular instance I18N is not strictly necessary, making the determination when I18N is actually mandatory, and when it's not, is somewhat cumbersome. Choosing to use I18N encoding for all non-English characters, as well as certain other special punctuation marks that have special meaning in certain contexts (such as =, but also ", and a few other things), is much simpler.
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