Email Signature Being Blocked

I recently implemented a standardized email signature at the company I am working at.  I am using remote images hosted on our website within the signature so that I can swap them out for fun themes for holidays and such.

I have run countless tests at mail-tester . com and all have come back positive.  With the exception of our mail server being listed on three spam lists since our email is through Gmail and that is out of our control.  

I have an issue where about two of our clients simply cannot receive email from us when sent with the signature.  Without the signature, the mail delivers just fine but with the signature it just doesn't even get delivered.. not in inbox or spam folder.  It's as if it was never sent.

I've tried everything from updating our website's SPF record and dkim signature to allow the Gmail server to send on our behalf (something that probably should have been setup long ago) and the problem still persists.

Can anybody think of any reason that our signatures would be blocked.. albeit by a very small percentage of our client's mail servers?

I did see the below question that was asked back in 2011 but my emails have passed every tester that I've found.
https://www.experts-exchange.com/questions/26732296/Email-Signature-being-blocked.html
AJ ArshemAsked:
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JohnBusiness Consultant (Owner)Commented:
am using remote images .... that I can swap them out for fun themes

That is undoubtedly the reason. Signatures can be a source of viruses and other malware. Tone them down to a level acceptable to all receivers and you should be fine.

I use just simple text signatures.
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Shaun VermaakTechnical Specialist/DeveloperCommented:
Are these images hosted on HTTP or HTTPS. First thing I would do is to host it with HTTPS
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AJ ArshemAuthor Commented:
Thank you Shaun and John,

I am hoping to be able to continue using the images.  Shaun, they are hosted on HTTP.  That very well may be a cause for the deliverability issues.  We do not currently have an SSL certificate for our site, although it is probably past due to get one.

Thanks!

AJ
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skullnobrainsCommented:
using remote images is considered by most antispam software as an indication of spamminess. some are more aggressive than others. additionally the links corresponding to those images might end up on an RHSBL which would produce many more rejections. and anyway many mail clients won't display these by default. you probably should use inline images and find a way to regenerate all signatures easily when needed. keep the images small.
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JohnBusiness Consultant (Owner)Commented:
Simple signatures are required for complete ability to send to anyone
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