I get this when I forward more than two or three emails.
An error occurred while sending mail. The mail server responded: 4.3.2 STOREDRV.ClientSubmit; sender thread limit exceeded. Please verify that your email address is correct in your account settings and try again.
An error occurred while sending mail. The mail server responded: 4.3.2 STOREDRV.ClientSubmit; sender thread limit exceeded. Please verify that your email address is correct in your account settings and try again.
Opaite Mbohovái (1)
I'm not sure if this will help, but for IMAP accounts you can specify the maximum number of server connections. Try going to Tools > Account Settings > [account name] > Server Settings > Advanced and decrease the value of "Maximum number of server connections to cache". HTH