Question: the outlook.ost has reached its maximum size?
Answer: OST files are Outlook offline data files. The OST file format has a maximum file size that depends on the version of Outlook you're running: Outlook 2010 and later support an OST file size of up to fifty gigabytes (50GB) Outlook 2007 and 2003 and earlier support an OST file size of up to twenty gigabytes (20GB)