You have two types of servers involved here. There is not gathering as such going on, just normal mail transfers.
mx2 are MTAs (Mail Transfer Agents). They are responsible for accepting email and delivering it towards the final destination. In the case of incoming email, they will forward it to
pop if that is the MDA (Message Delivery Agent) for the user. All other email will will be forwarded to an appropriate server if possible. These servers will store email and retry delivery if necessary.
pop is an MDA and is responsible for storing and delivering email for the user. As an MDA it will not forward email to an MTA. It appears to be a POP server and will have a message store for each user it is responsible for. When the user's UA (User Agent) connects to
pop they can only read email which has already been delivered. It is possible that
mx2 will be holding email which could not be delivered for some temporary reason (
pop is down, user has exceeded quota, etc.).
Users of pop will likely have been given an SMTP address to be used when sending email. This server will function as an MSA (Mail Submission Agent) when the users connect. Its task is to authenticate the user (local users may not need to login), and add any required headers that the user's UA (User Agent) failed to add. The message will then be sent to the appropriate MX or possibly directly to an MDA.
It is common for MX servers to act as an MDA or MSA. Here are some diagrams which might help understand the routing for incoming email.
email ---+---> mx1 ---+---> pop ---> UA
+---> mx2 ---+
In your example, email destined for
pop is deliverer either
mx2. Which ever MX receives the mail forwards it to
pop when possible (usually immediately).
pop stores the mail until the user's UA connects and reads the mail. Depending on the
pop setting email is deleted when read. Neither
mx2 needs to be up when email is read. However, as
pop is the MDA and has the email to be read, it must be up when you want to read email. The user will read only email which has been delivered to
pop and will not be fetching or gathering email from
UA ---> MSA ---+---> MX ----+---> MXs ---+
+------------+------------+---> MDA ---> UA
Outgoing email goes from the user's UA to
smtp, its MSA. The MSA sends the email to an MX (possibly
mx2), or directly to an MDA like
pop. The MX may tranfer the email through additional MXs until it reaches and MDA which accepts the email for delivery. If at any step along the way, the email is determined to be undeliverable, it will be returned to sender if possible. At some point a UA may be used to read the email.