Many/most free and even paid email clients & webmail services may run into issues with bulk recipient lists, because of spamming concerns.
I've used Thunderbird for many, many years (practically from its inception) under at least XP, Vista, 7 and 10, with no problems with BCC.
I've found it to be robust, stable, customizable and reliable.
Having said that, there are several other paid and free clients out there (not counting webmail, such as gmail, yahoo! mail, etc.)
None will be perfect on every system and to suit every user.
Each has its advantages and disadvantages.
And, for your bulk mailing needs, you may need a paid program.
Here are a few resources to get you started, offered "as is", since I have not personally used any of them except Thunderbird: