It is not really a bug, more like a lack of a feature.
I know that Aquamail is making sure that at least one of the address fields is non-empty.
I discovered that Aquamail doesn't perform any tests for checking that the content is a
legitimately-looking e-mail address. I am talking about a simple test for the presence of "@" (and maybe some other simple consistency checks, such as absence of spaces breaking up otherwise a legitimate address.
I had one letter entered in the address field, and the app attempted to send it, predictably resulting in an error.
While many programs don't do that, I'd think it is an easy and relatively simple test to do. I believe I've seen some programs that do that, but I don't remember off the top of my head.
While we are at it, - may I propose that a "nudge" about an empty Subject would also be helpful.
Even many "historic" e-mail programs, such as
elm, pine/alpine ... warn about that.
(And Thunderbird goes further, helping to prevent a common mistake, forgetting to attach an attachment that is mentioned in the text. Even though I appreciate that, I am not suggesting to go that far here. )
And one more thought in relation to the first issue above.
I have contacts that consist of just one letter in the name.
There are several reasons, but let me give you two:
1. Some people in the US that use they initial(s) instead of the name.
2. For privacy reasons (some Androids apps (including those by Google) insist on snooping into Contacts).
Unfortunately, Aquamail doesn't start picking any contacts until you enter two letters.
I remember your response in the discussion about this a few months ago, and I understand why you might be reluctant to start offering e-mail addresses with just one letter. But I wonder if you would consider doing that for the Names in the Contacts? I don't know if that would be too complicated to implement. If it is, don't worry, I have some workarounds (e.g. entering the first two letters of the e-mail address for these people, which requires me remembering those).