Stuck emails

It depends on the load on the server. If a huge amount of mail reaches the server , it may take hours till he finishes the delivery.  I the server is relatively free , it is instantaneous.

We can't say that it depends only on the server.

mostly bandwidth utilisation of you network or home PC can slow down the mail arrivals, as well as the attachments with the mail too.

Try sending "n" no. of plain text mails to yourself or to your friend, and see how long it takes to reach you or UR friend's Inbox.