you must change something on the Active Sync policy for your Exchange Server.

Look her

https://docs.microsoft.com/en-us/exchange/mail-flow/message-size-limits?view=exchserver-2019

%windir%\system32\inetsrv\appcmd.exe set config “Default Web Site/Microsoft-Server-ActiveSync/” -section:system.webServer/serverRuntime /uploadReadAheadSize:31457280

Restart iis and all is fine