Greetings,
I am having the same problem as this 2021 post: Spell check & capitalised first letter of sentence and Spelling check: first word in sentence "wrong" as it starts with a capital letter
case 75485 seems to have resolved spontaneously, so a fix was never outlined. and there is no posted response much less a fix to: 96561.?
to recap: I have used eM Client for years on many computers, but suddenly on my laptop, (current version), the spell checker highlights any capitalized word despite correct spelling.
Originally set to English (United Kingdom) I have tried “default” language setting and English (US) but this has made no difference
Does anyone have a fix?
I am having the exact same problem. It just started after the most recent update was installed.
Doesn’t look like anyone at eM cares about this… I have two users, husband and wife, who have the same exact version and identical installations (as far as I can see, comparing settings) but one of them has this symptom (Windows 10) and the other doesn’t (Win11) 
OK looks like we have a resolution here. Well, this worked for me, anyway:
Three bars menu → Settings → General → Spell-Checker
- Default settings: Language: Application language
- Mail Spell Checker: Language: Use Default Language Settings
- [I never use Chat but I assume Chat Spell-Checker Language should be set the same]
Now make sure your WIndows language settings are correct for the settings you want used, e.g. don’t leave it on English (US) if you don’t want colour, defence, and kidnapped flagged
