Iphone smtp port 25 or 587 change without manual action

setup of Mail,contact, calendar-><my comanyaccount>->IMAP <account name-->Server mail Out--> Main server-> Label PORT SERVER

iOS 9.2.1 ( nor is last udate)

PORT SERVER it's set normal set to 587 and use SSL with password. This setup run good when go out of company. When Iphone contact internal WIFI coverage Iphone change port to 25 and required manual setup for reset PORT SERVER to 25.

When travel to my home this change of port it's same and required manual reset to 587. The activity it's random and not any day required change.

This Iphone is join apple ID with mac Air with El Captain and into setup of Mail i have disable check SMTP port for stable fix 587 send mail port.

It' s available flags on Iphone for lock SMTP port 587, 25 or other....

Think for solve the problem.