Problems Sending or Receiving Email Due To Too Many IMAP Accounts
If you have lots of IMAP accounts on the same server, you might be running into problems because Postbox is trying to open too many connections.
To fix this:
1) Go into "Preferences | Accounts | <each IMAP account you have> | Advanced"
2) Change the "Maximum number of server connections to cache" from 5 to 1
3) Restart Postbox