It seems that sometimes message lists do not fetch new E-Mail when they are unfocused. For example, in a multi-account setup, when Account_A is focused and Account_B receives E-Mail, switching to Account_B (with :ct Account_B) will sometimes show the message list without the newly received email.

The same might happen for example when receiving an E-Mail while reading another in the same account and folder. After closing the E-Mail being read the message list does not show the newly received E-Mail.