Please check the IIS settings:
1. Open properties of OWA visual directory
2. In Directory Security tab, under Secure communications, click Edit. Make sure that the box Ignore client certificates is sleeted
3. Restart IIS when it’s possible
NOTE: If the issue persists, move to the next step….
4. Open ESM, run the following command to specify the maxsendsize value for a user who has this problem
Set-mailbox <UserAlias> -maxsendsize: <value>