Gmail problem

I have emailed support directly about this but the silence is deafening so thought I'd try here. I don't expect to get a quick response from any Zoho employee (when does Zoho do anything quickly?) but am hoping some other user might have come across this problem and know the solution.

I have a custom button with a custom function. The custom function includes the sendmail function. The custom function works fine - apart from some times. The function sometimes fails if any of the email addresses in the sendmail function are to a gmail account.

I've done extensive testing - executing the custom function via script builder. Using the same record id each time (where one of the email addresses in the sendmail function will be a gmail address), it will run successfully several times in a row and then it will fail. Error message returned is telling me it's due to an invalid email address, which is rubbish because it worked previous time!

It definitely seems to be gmail specific but why is it working sometimes and failing other times??

The other extremely annoying thing I've discovered whilst doing this is that if you send emails using sendmail function they are not recorded anywhere!!! (I have to use a custom function because I need to add cc addresses and you can't do this with email templates!) So there's no record of the sent email anywhere!!!!!

And more worrying still is that whilst searching forum I found several posts indicating that if emails sent via CRM bounce for any reason e.g. email address is wrong then you never get to hear about the bounce!! This one small thing could result in us losing a HUGE amount of potential income!

Here are some of the posts relating to this:
https://help.zoho.com/portal/en/community/topic/unfuckingbelievable
https://help.zoho.com/portal/en/community/topic/customer-did-not-receive-email-sent-out-via-crm
https://help.zoho.com/portal/en/community/topic/rejected-emails-not-being-seen-by-sender

I've spent months getting Zoho ready to go live and the day we do the very first problem we have is not being able to use sendmail function with gmail accounts. We have hundreds of clients and agents with gmail accounts. The whole point of moving to Zoho was so that we could automate a lot of tasks. I'm beginning to wish I hadn't bothered.
It would help a huge amount if there was proper, responsive support! Every time I reach out for support I end up going round in circles getting no where.