Yes Linux mail servers are used and trusted globally and their popularity is increasing by the day.
Linux based mail servers are widely used because:

• They make you the true owner of your mail server ensuring complete control and giving you freedom
• They are characterised by stability, security and scalability
• They are extremely scalable through clustering and replications
• They are customisable to suit your business needs and processes
• They are suited for high concurrent usage
• Linux based solutions are built to fully exploit the features of the hardware components hence the linux based mail servers are 'performance' oriented
• They don't involve complicated licenses
• They are neither exorbitantly priced (mostly they are free to download) nor have hidden charges. The cost thus revolves around the support contract and is again very reasonable