Some of this is covered here.

Some other factors that are specific to emails include:
  • Are you rebuilding the group at send time?
  • Are you scheduling the email or sending immediately?
  • Are the accounts that are taking longer gmail accounts?

These all can play a factor in how long it can take to receive an email.  If the group rebuilds at send, and the send is immediate, that means all of the processing has to take place then, rather than ahead of time if it is scheduled.

Also, some email providers can take up to 24 hours to deliver an email, especially Gmail. We continue to work with the email providers to ensure the best delivery possible but we do not have any control over those providers and they can block or delay emails as they choose.