Use Phpmailer send content inside write css,css inside use background url (XXX), finally parse out URL () inside is empty

Use Phpmailer sent content inside wrote css,css inside used to background url (XXX), finally parse out URL () inside is empty, is how to go? Who has had the same experience?
I don't know where to filter it.

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e-Mail needs to write the URL of the specific address, the URL can be accessed through the external network

e-Mail needs to write the URL of the specific address, the URL can be accessed through the external network I write a specific address, I mean the whole change empty, pro. It seems to be a filtering process, but I don't know where to deal with it.

e-Mail needs to write the URL of the specific address, the URL can be accessed through the external network I write a specific address, I mean the whole change empty, pro. It seems to be a filtering process, but I don't know where to deal with it.
This has not met, I have done before the e-mail also contains background, the same URL, but did not encounter this situation
I thought maybe it was some of your other styles that affected that piece.
I have encountered such a problem, is the mail sent out, but the style is not, the reason is that some of the style to affect, you can optimize your style

e-Mail needs to write the URL of the specific address, the URL can be accessed through the external network I write a specific address, I mean the whole change empty, pro. It seems to be a filtering process, but I don't know where to deal with it.
This has not met, I have done before the e-mail also contains background, the same URL, but did not encounter this situation
I thought maybe it was some of your other styles that affected that piece.
I have encountered such a problem, is the mail sent out, but the style is not, the reason is that some of the style to affect, you can optimize your style mailbox itself, a change of mailbox account is OK.

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