Formula to locate to Output Characters after the Last "."
MID(
MID(
(MID(Username__c, FIND('@', Username__c)+1, 100)),
(Find (".",(MID(Username__c, FIND('@', Username__c)+1, 100))))+1,
100),
FIND(".",
MID(
(MID(Username__c, FIND('@', Username__c)+1, 100)),
(Find (".",(MID(Username__c, FIND('@', Username__c)+1, 100))))+1,
100))+1,
100
)