Unable to learn that they are sending packets are too big for some paths, the servers are unable to send data to some users.  To overcome this they need to accept appropriate ICMP types or allow fragmentation.

We have a list of some web servers with broken path MTU discovery.

Path MTU Discovery

If a packet starts out on a network segment with a large MTU, it may arrive at a link with a smaller MTU and be too big to fit.  Most servers are on segments with large MTUs, but it is increasingly common for internet users to be connected via links with reduced MTUs, so it is becoming common for some packets to be too big.

How the problem of oversize packets has been handled has evolved considerably over time.  The original approach was to send only small packets corresponding to the TCP/IP default MTU (576 bytes).  (To this day, a sending system needs permission from the receiving system to send larger packets, but that permisssion is given as a matter of routine.)

For some packets, especially those sent by older equipment, an oversize packet can be sent by breaking it into fragments and sending the fragments as smaller packets.  The fragments can be reassembled downstream to reconstruct the original large packet, but this packet fragmentation has several problems involving both efficiency and security.

Newer servers try to optimize their transmissions by discovering the path MTU and sending packets of the maximum size when there's enough data to fill them. The procedure for doing this was standardized and published as RFC 1191 in 1990, but it did not become widely deployed until years later. By mid 2002, 80% to 90% of computers on the internet used path MTU discovery.