Hello,
Being an email marketing specialist on a daily basis (my job being just as stated: Email MK Specialist), i can fix your newsletter so it will work with any email framework or client application known in order to avoid the spam folder.
First of all i will review the code behind your newsletters (which is an absolute must). Most of the newsletters reaching the spam folder have a buggy code and it needs re-coding.
Secondly i will check the content and suggest changes if needed (you need to have relevant content, without spam words or phrasing in it)
Third i will check the email settings of the SendGrid framework in order to optimize it for future email blasts.
Thank you for considering my bid,
Silverleaf