The usual reason is that your PC, Mac or mobile device is using POP3 to check for new mail while SpamDrain is using IMAP. Some mail providers lock out any POP3 client accesses while an IMAP client is connected.
To fix the problem you could try to switch SpamDrain into using POP3 instead of IMAP. Log on to spamdrain.com/app.html#settings. Click your e-mail address in the list of filtered e-mail addresses. Switch to POP3 in the Type list, change the server name in Server for incoming e-mail box to your mail provider's POP3 server and click Save.