problem with yahoo mail

hi
my users connect to rb433 with pppoe and hotspot
mikrotik cache server is disabled

but when opening yahoo mail in clinet the page hanging

i tested it behind mikrotik and all is ok . note that when i connect vpn from client to a server (no to my mikrotik) the problem will be solved.

whats the problem?

it solved by decreasing MTU and MRU to 1200

Thank you ! This has fixed my problem