I am not quite sure what you mean by "sign in to it." eM Client, like other email client programs, is not signed in to, per se. You set up your e-mail account or accounts like you do in virtually any other these days, and if it's with one of the major or many of the minor service providers all you will do is provide your e-mail address and password and it will do the rest.
ALT+M,A or just ALT+A opens the Accounts dialog, and the add account button is at the top. I have to SHIFT+TAB several times to land on it.
eM Client is still not 100% accessible, though they have taken feedback during beta phases and seem very receptive to more in an effort to make it more accessible. I believe that several group members here worked with it during the last beta and gave feedback. A good example of lack of perfect accessibility is once you activate the Add Account button. The dialog to add account opens, and you are in the edit box for your e-mail address, but NVDA just says EDIT. It appears that the box is not labeled such that the screen reader can announce it.
I do think there are several members now using eM Client as their daily driver for e-mail, so I'm imagining the main interface is 100% accessible or very close to it. But I'll leave it to those who do use it to identify it's good, and bad, points. I only work with it when doing testing.
Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042
[Regarding the Supreme Court refusing to hear the case brought by Texas to overturn the votes certified by 4 states:] Pleased with the SCOTUS ruling, but also immediately slightly terrified of where this crazy train goes next. We should know by now there’s a bottomless supply of crazy.
~ Brendan Buck, former adviser to Speakers of the House Paul Ryan and John Boehner