Although I am not sure about your existing email service provider but it will better to go with any transnational email clients like Mandrill, Sendinblue, Amazon SES etc; Basically those transnational email clients improves customer confidence; And Lower Burdens on other customer service channels, better branding opportunities adds many revenues to the business and importance of the email; So undoubtedly, those will be very helpful to prevent spam messaging as well;
Basically What you say in your emails is incredibly important. If the junk mail filter has identified one or more keywords that spammers repeatedly use in the message, the email reaches the recipient's spam folder.
May be, You are sending certain attachments or only pictures with the message that are not needed; In general, .jpg, .gif, .png and .pdf attachments are safe to send, provided you include some content in the email as well. However, executable attachments such as .exe, .zip, .swf, etc. should be avoided entirely.
Here is the overall tasks:
Check the email provider details; 2 to 3 hours;
Check content of the email and check if you are using Re or FWD or Click here etc type of content there; 2 hours;
Check for attachment type; 2 hours;
Recipients ISP set up checking takes 4-5 hours
Attached file size checking takes 1.5 hours;
As soon as, find out the route of issue, will rectify it; That will be less time work for me;
Need to check the existing email client before give you exact estimate;