You have two types of servers involved here. There is not gathering as such going on, just normal mail transfers.
mx1
and 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 mx1
and 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 mx1
or 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 mx1
nor 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 mx1
or mx2
.
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 mx1
or 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.