Describe the bug
Upgrading Mimekit from 4.14.0 to 4.15.0 introduced high memory usage.

Platform (please complete the following information):

• OS: Linux
• .NET Runtime: 9.0
• .NET Framework: Net Core
• MimeKit Version: 4.15.0

To Reproduce
Use version 4.15.0

Expected behavior
Increased memory usage on service leading to OOM