Emails not being received by clients recently

Emails not being received by clients recently

We have been using Zoho for a total of 11 of our company accounts for last several months and have been mostly happy with the service. However, recently we have been getting a lot of feedback from out clients that they are not getting the emails we send them. This is mostly with new clients with gmail ids. Most of the clients say the email ends up in their spam and the others complain they dont receive them at all. 

This is mostly one on one correspondence with clients with little or no pictures apart from our signatures. 

This makes us look really bad and most importantly affects our business. Please get this issue sorted ASAP. 

Regards, 

Muhammad