Zimbra external account trouble

Hello,

This question is with regards to the Zimbra mail server. My version is Zimbra 8.7.7_GA_1787 (build 20170410133400) and couple of users have their external accounts located on a different email hosting servers like Gmail, A2hosting and Liquidweb.

We have added their emails as the external account in the Zimbra online webmail as POP. Also we put below command to POP emails in every 60 seconds,

zmprov mds user@domain.com ExternalIM zimbraDataSourcePolling Interval 60

However all users are complaining that they are receiving the email very late and sometime they do not receive.

So we remove the external account and then add again. But when lot of users complain the same then we restart the zmcontrol services.

Is there any permanent solution for this because it is happening very frequently and I can not restart the services every time....