I can receive emails, but cannot send any, why? For the spamming related reasons your Internet provider is blocking you from using anyone's outgoing mail servers but their own. You will still be able to send emails, but only by using your ISP's outgoing mail server. Every ISP's outgoing mail address is different, but for the most part they will be mail.yourispname.com If this doesn't help you will need to call your Internet Service Provider to find out what their outgoing mail server is.

I can send emails, but cannot receive any, why? Either your domain is out of disk space, or your e-mail address itself is out. Increase the amount of disk space to receive email again.

I cannot login even though I'm using correct details, why?

Most of our servers have a 60 checks per hour limit. If you hit this limit in any hour you'll be locked out for the rest of the hour. You will need to click Tools » Options » General  and disable where it says Check for new messages every xx minutes. If you do not want to disable it, just set it to check less often. Keep in mind the more computers you have auto checking or checking the mail, the quicker you will hit this limit.

Why do e-mails with attachments not work? Some attachments are blocked by our server (.bat .src .exe). The reason for this is to prevent your mailbox being flooded with viruses. A few attachments that are fine without zipping up are: .gif .jpg .html .pdf. You cannot turn off this option. If you want to send large files you need to send them as a zip or upload them to your website and give out the link. You could even password protect so the public cannot view or access the file.