Yes there is a problem with other sites not accepting tag addressing even though it is acceptable under RFC papers. I have not had problems using it with gmail (I think there was an initial problem where some DH users had the spam detection feature set up at DH although I thought this had been fixed.) but some sites do reject it. I think it occurs when email addresses are regexd, or otherwise filtered for inclusion into a database record, and the plus sign is seen as an error in the email address when it is not.
All you can really do is ask them to amend their systems to correctly accept plus tagged addresses.
Opinions are my own views, not DreamHosts'.
I am NOT a DreamHost employee OK!! :@
You act on my advice at your own risk!